Typically, your repository interface extends Repository, CrudRepository, or PagingAndSortingRepository. Alternatively, if you do not want to extend Spring Data interfaces, you can also annotate your repository interface with @RepositoryDefinition. Extending CrudRepository exposes a complete set of methods to manipulate your entities. If you prefer to be selective about the methods being exposed, copy the methods you want to expose from CrudRepository into your domain repository.

The following example shows how to selectively expose CRUD methods (findById and save, in this case):

In the prior example, you defined a common base interface for all your domain repositories and exposed findById(…) as well as save(…).These methods are routed into the base repository implementation of the store of your choice provided by Spring Data (for example, if you use JPA, the implementation is SimpleJpaRepository), because they match the method signatures in CrudRepository. So the UserRepository can now save users, find individual users by ID, and trigger a query to find Users by email address.

Using a unique Spring Data module in your application makes things simple, because all repository interfaces in the defined scope are bound to the Spring Data module. Sometimes, applications require using more than one Spring Data module. In such cases, a repository definition must distinguish between persistence technologies. When it detects multiple repository factories on the class path, Spring Data enters strict repository configuration mode. Strict configuration uses details on the repository or the domain class to decide about Spring Data module binding for a repository definition:

The following example shows a repository that uses module-specific interfaces (JPA in this case):

The following example shows a repository that uses generic interfaces:

The following example shows a repository that uses domain classes with annotations:

The following bad example shows a repository that uses domain classes with mixed annotations:

Repository type details and distinguishing domain class annotations are used for strict repository configuration to identify repository candidates for a particular Spring Data module. Using multiple persistence technology-specific annotations on the same domain type is possible and enables reuse of domain types across multiple persistence technologies. However, Spring Data can then no longer determine a unique module with which to bind the repository.

The last way to distinguish repositories is by scoping repository base packages. Base packages define the starting points for scanning for repository interface definitions, which implies having repository definitions located in the appropriate packages. By default, annotation-driven configuration uses the package of the configuration class. The base package in XML-based configuration is mandatory.

The following example shows annotation-driven configuration of base packages:

The following strategies are available for the repository infrastructure to resolve the query. With XML configuration, you can configure the strategy at the namespace through the query-lookup-strategy attribute. For Java configuration, you can use the queryLookupStrategy attribute of the Enable${store}Repositories annotation. Some strategies may not be supported for particular datastores.

The query builder mechanism built into the Spring Data repository infrastructure is useful for building constraining queries over entities of the repository.

The following example shows how to create a number of queries:

Parsing query method names is divided into subject and predicate. The first part (find…By, exists…By) defines the subject of the query, the second part forms the predicate. The introducing clause (subject) can contain further expressions. Any text between find (or other introducing keywords) and By is considered to be descriptive unless using one of the result-limiting keywords such as a Distinct to set a distinct flag on the query to be created or Top/First to limit query results.

The appendix contains the full list of query method subject keywords and query method predicate keywords including sorting and letter-casing modifiers. However, the first By acts as a delimiter to indicate the start of the actual criteria predicate. At a very basic level, you can define conditions on entity properties and concatenate them with And and Or.

The actual result of parsing the method depends on the persistence store for which you create the query. However, there are some general things to notice:

Property expressions can refer only to a direct property of the managed entity, as shown in the preceding example. At query creation time, you already make sure that the parsed property is a property of the managed domain class. However, you can also define constraints by traversing nested properties. Consider the following method signature:

Assume a Person has an Address with a ZipCode. In that case, the method creates the x.address.zipCode property traversal. The resolution algorithm starts by interpreting the entire part (AddressZipCode) as the property and checks the domain class for a property with that name (uncapitalized). If the algorithm succeeds, it uses that property. If not, the algorithm splits up the source at the camel-case parts from the right side into a head and a tail and tries to find the corresponding property — in our example, AddressZip and Code. If the algorithm finds a property with that head, it takes the tail and continues building the tree down from there, splitting the tail up in the way just described. If the first split does not match, the algorithm moves the split point to the left (Address, ZipCode) and continues.

Although this should work for most cases, it is possible for the algorithm to select the wrong property. Suppose the Person class has an addressZip property as well. The algorithm would match in the first split round already, choose the wrong property, and fail (as the type of addressZip probably has no code property).

To resolve this ambiguity you can use _ inside your method name to manually define traversal points. So our method name would be as follows:

Because we treat the underscore character as a reserved character, we strongly advise following standard Java naming conventions (that is, not using underscores in property names but using camel case instead).

To handle parameters in your query, define method parameters as already seen in the preceding examples. Besides that, the infrastructure recognizes certain specific types like Pageable and Sort, to apply pagination and sorting to your queries dynamically. The following example demonstrates these features:

The first method lets you pass an org.springframework.data.domain.Pageable instance to the query method to dynamically add paging to your statically defined query. A Page knows about the total number of elements and pages available. It does so by the infrastructure triggering a count query to calculate the overall number. As this might be expensive (depending on the store used), you can instead return a Slice. A Slice knows only about whether a next Slice is available, which might be sufficient when walking through a larger result set.

Sorting options are handled through the Pageable instance, too. If you need only sorting, add an org.springframework.data.domain.Sort parameter to your method. As you can see, returning a List is also possible. In this case, the additional metadata required to build the actual Page instance is not created (which, in turn, means that the additional count query that would have been necessary is not issued). Rather, it restricts the query to look up only the given range of entities.

You can limit the results of query methods by using the first or top keywords, which you can use interchangeably. You can append an optional numeric value to top or first to specify the maximum result size to be returned. If the number is left out, a result size of 1 is assumed. The following example shows how to limit the query size:

The limiting expressions also support the Distinct keyword for datastores that support distinct queries. Also, for the queries that limit the result set to one instance, wrapping the result into with the Optional keyword is supported.

If pagination or slicing is applied to a limiting query pagination (and the calculation of the number of available pages), it is applied within the limited result.

Query methods that return multiple results can use standard Java Iterable, List, and Set. Beyond that, we support returning Spring Data’s Streamable, a custom extension of Iterable, as well as collection types provided by Vavr. Refer to the appendix explaining all possible query method return types.

As of Spring Data 2.0, repository CRUD methods that return an individual aggregate instance use Java 8’s Optional to indicate the potential absence of a value. Besides that, Spring Data supports returning the following wrapper types on query methods:

Alternatively, query methods can choose not to use a wrapper type at all. The absence of a query result is then indicated by returning null. Repository methods returning collections, collection alternatives, wrappers, and streams are guaranteed never to return null but rather the corresponding empty representation. See “Repository query return types” for details.

You can process the results of query methods incrementally by using a Java 8 Stream<T> as the return type. Instead of wrapping the query results in a Stream, data store-specific methods are used to perform the streaming, as shown in the following example:

You can run repository queries asynchronously by using {spring-framework-docs}/integration.html#scheduling[Spring’s asynchronous method running capability]. This means the method returns immediately upon invocation while the actual query occurs in a task that has been submitted to a Spring TaskExecutor. Asynchronous queries differ from reactive queries and should not be mixed. See the store-specific documentation for more details on reactive support. The following example shows a number of asynchronous queries:

Each Spring Data module includes a repositories element that lets you define a base package that Spring scans for you, as shown in the following example:

In the preceding example, Spring is instructed to scan com.acme.repositories and all its sub-packages for interfaces extending Repository or one of its sub-interfaces. For each interface found, the infrastructure registers the persistence technology-specific FactoryBean to create the appropriate proxies that handle invocations of the query methods. Each bean is registered under a bean name that is derived from the interface name, so an interface of UserRepository would be registered under userRepository. The base-package attribute allows wildcards so that you can define a pattern of scanned packages.

You can also trigger the repository infrastructure by using a store-specific @Enable${store}Repositories annotation on a Java configuration class. For an introduction to Java-based configuration of the Spring container, see {spring-framework-docs}/core.html#beans-java[JavaConfig in the Spring reference documentation].

A sample configuration to enable Spring Data repositories resembles the following:

You can also use the repository infrastructure outside of a Spring container — for example, in CDI environments. You still need some Spring libraries in your classpath, but, generally, you can set up repositories programmatically as well. The Spring Data modules that provide repository support ship with a persistence technology-specific RepositoryFactory that you can use, as follows:

To enrich a repository with custom functionality, you must first define a fragment interface and an implementation for the custom functionality, as follows:

The implementation itself does not depend on Spring Data and can be a regular Spring bean. Consequently, you can use standard dependency injection behavior to inject references to other beans (such as a JdbcTemplate), take part in aspects, and so on.

Then you can let your repository interface extend the fragment interface, as follows:

Extending the fragment interface with your repository interface combines the CRUD and custom functionality and makes it available to clients.

Spring Data repositories are implemented by using fragments that form a repository composition. Fragments are the base repository, functional aspects (such as QueryDsl), and custom interfaces along with their implementations. Each time you add an interface to your repository interface, you enhance the composition by adding a fragment. The base repository and repository aspect implementations are provided by each Spring Data module.

The following example shows custom interfaces and their implementations:

The following example shows the interface for a custom repository that extends CrudRepository:

Repositories may be composed of multiple custom implementations that are imported in the order of their declaration. Custom implementations have a higher priority than the base implementation and repository aspects. This ordering lets you override base repository and aspect methods and resolves ambiguity if two fragments contribute the same method signature. Repository fragments are not limited to use in a single repository interface. Multiple repositories may use a fragment interface, letting you reuse customizations across different repositories.

The following example shows a repository fragment and its implementation:

The following example shows a repository that uses the preceding repository fragment:

The approach described in the preceding section requires customization of each repository interfaces when you want to customize the base repository behavior so that all repositories are affected. To instead change behavior for all repositories, you can create an implementation that extends the persistence technology-specific repository base class. This class then acts as a custom base class for the repository proxies, as shown in the following example:

The final step is to make the Spring Data infrastructure aware of the customized repository base class. In Java configuration, you can do so by using the repositoryBaseClass attribute of the @Enable${store}Repositories annotation, as shown in the following example:

A corresponding attribute is available in the XML namespace, as shown in the following example:

Querydsl is a framework that enables the construction of statically typed SQL-like queries through its fluent API.

Several Spring Data modules offer integration with Querydsl through QuerydslPredicateExecutor, as the following example shows:

To use the Querydsl support, extend QuerydslPredicateExecutor on your repository interface, as the following example shows:

The preceding example lets you write type-safe queries by using Querydsl Predicate instances, as the following example shows:

Spring Data modules that support the repository programming model ship with a variety of web support. The web related components require Spring MVC JARs to be on the classpath. Some of them even provide integration with Spring HATEOAS. In general, the integration support is enabled by using the @EnableSpringDataWebSupport annotation in your JavaConfig configuration class, as the following example shows:

The @EnableSpringDataWebSupport annotation registers a few components. We discuss those later in this section. It also detects Spring HATEOAS on the classpath and registers integration components (if present) for it as well.

Alternatively, if you use XML configuration, register either SpringDataWebConfiguration or HateoasAwareSpringDataWebConfiguration as Spring beans, as the following example shows (for SpringDataWebConfiguration):

If you work with the Spring JDBC module, you are probably familiar with the support for populating a DataSource with SQL scripts. A similar abstraction is available on the repositories level, although it does not use SQL as the data definition language because it must be store-independent. Thus, the populators support XML (through Spring’s OXM abstraction) and JSON (through Jackson) to define data with which to populate the repositories.

Assume you have a file called data.json with the following content:

You can populate your repositories by using the populator elements of the repository namespace provided in Spring Data Commons. To populate the preceding data to your PersonRepository, declare a populator similar to the following:

The preceding declaration causes the data.json file to be read and deserialized by a Jackson ObjectMapper.

The type to which the JSON object is unmarshalled is determined by inspecting the _class attribute of the JSON document. The infrastructure eventually selects the appropriate repository to handle the object that was deserialized.

To instead use XML to define the data the repositories should be populated with, you can use the unmarshaller-populator element. You configure it to use one of the XML marshaller options available in Spring OXM. See the {spring-framework-docs}/data-access.html#oxm[Spring reference documentation] for details. The following example shows how to unmarshall a repository populator with JAXB:

We provide @CreatedBy and @LastModifiedBy to capture the user who created or modified the entity as well as @CreatedDate and @LastModifiedDate to capture when the change happened.

As you can see, the annotations can be applied selectively, depending on which information you want to capture. The annotations capturing when changes were made can be used on properties of type Joda-Time, DateTime, legacy Java Date and Calendar, JDK8 date and time types, and long or Long.

Auditing metadata does not necessarily need to live in the root level entity but can be added to an embedded one (depending on the actual store in use), as shown in the snipped below.

In case you do not want to use annotations to define auditing metadata, you can let your domain class implement the Auditable interface. It exposes setter methods for all of the auditing properties.

There is also a convenience base class, AbstractAuditable, which you can extend to avoid the need to manually implement the interface methods. Doing so increases the coupling of your domain classes to Spring Data, which might be something you want to avoid. Usually, the annotation-based way of defining auditing metadata is preferred as it is less invasive and more flexible.

In case you use either @CreatedBy or @LastModifiedBy, the auditing infrastructure somehow needs to become aware of the current principal. To do so, we provide an AuditorAware<T> SPI interface that you have to implement to tell the infrastructure who the current user or system interacting with the application is. The generic type T defines what type the properties annotated with @CreatedBy or @LastModifiedBy have to be.

The following example shows an implementation of the interface that uses Spring Security’s Authentication object:

The implementation accesses the Authentication object provided by Spring Security and looks up the custom UserDetails instance that you have created in your UserDetailsService implementation. We assume here that you are exposing the domain user through the UserDetails implementation but that, based on the Authentication found, you could also look it up from anywhere. :leveloffset: -1

A high level look of the architecture looks like:

Those coming from other Spring Data projects or are familiar with ORM products like JPA or Hibernate may quickly recognise this architecture.

Spring Data Neo4j is largely broken up into two main components:

It is recommended SDN developers also familiarise themselves with Neo4j-OGM. The Neo4j-OGM reference documentation has been reproduced after this section for convenience.

To create a Spring Boot project simply go to https://start.spring.io and specify a group and artifact like: org.spring.neo4j.example and demo. In the Dependencies box type: "Neo4j". You can also add any other Spring support like "Web" etc. Once you are satisfied with your dependencies hit the generate button, download the zip and unzip into your workspace.

To create a Spring project in STS go to File → New → Spring Template Project → Simple Spring Utility Project → press Yes when prompted. Then enter a project and a package name such as org.spring.neo4j.example.

Then add the following to pom.xml dependencies section.

Also change the version of Spring in the pom.xml to be

Spring Data Neo4j projects can be built using Maven, Gradle or any other tool that supports Maven’s repository system.

There is a github repository with several examples that you can download and play around with to get a feel for how the library works.

For most applications the following configuration is all that’s needed to get up and running.

Here we wire up a SessionFactory configured from defaults. We can change these defaults by providing an ogm.properties file at the root of the classpath or by passing in a org.neo4j.ogm.config.Configuration object. The last infrastructure component declared here is the Neo4jTransactionManager. We finally activate Spring Data Neo4j repositories using the @EnableNeo4jRepositories annotation. If no base package is configured it will use the one the configuration class resides in.

Note that you will have to activate @EnableTransactionManagement explicitly to get annotation based configuration at facades working as well as define an instance of this Neo4jTransactionManager with the bean name transactionManager. The example above assumes you are using component scanning.

To allow your query methods to be transactional simply use @Transactional at the repository interface you define.

The SessionFactory is needed by SDN to create instances of org.neo4j.ogm.session.Session as required. When constructed, it sets up the object-graph mapping metadata, which is then used across all Session objects that it creates. As seen in the above example, the packages to scan for domain object metadata should be provided to the SessionFactory constructor.

Your application typically connects only to one instance of Neo4j. Therefore there is usually only one SessionFactory per application. This SessionFactory should be configured in such a way that it scans all necessary packages. @EnableNeo4jRepositories detects one single bean of type SessionFactory out of the box and you don’t need to configure anything more.

In cases however where your setup requires the application to connect to multiple different instances of Neo4j, SDN allows you to specify that instance in @EnableNeo4jRepositories as follows:

In cases like this SDN also supports multiple declarations of @EnableNeo4jRepositories that target different packages. Our recommendation however is connecting one application to one instance. If your domains are so different that they are stored in different databases, a microservices approach might be a better fit to your application.

An OGM (Object Graph Mapper) maps nodes and relationships in the graph to objects and references in your domain model. Object instances are mapped to nodes while object references are mapped using relationships, or serialized to properties. JVM primitives are mapped to properties of a node or a relationship. An OGM lets you create a view of your database based on your domain model. It also provides the flexibility to supply custom queries in cases where the queries generated by Neo4j-OGM are insufficient.

The OGM can be thought of as analogous to Hibernate or JPA. Users should have an understanding of what an object mapper does when using this guide.

While SDN’s repositories will cover a majority of user scenarios, sometimes they don’t offer enough options. Neo4j-OGM’s Session offers a convenient API to interact more direct with a Neo4j graph database. SDN now allows you to wire up the Neo4j-OGM Session directly into your Spring managed beans.

For Spring Data Neo4j, low level operations are handled by Neo4j-OGM Session. Basic operations are limited to CRUD operations on entities and executing arbitrary Cypher queries; more low-level manipulation of the graph database is not possible.

Given that the latest version of the framework is driven by Cypher queries alone, there’s no way to work directly with Node and Relationship objects, especially in remote server mode. Similarly, the traverse() method has disappeared, again because the underlying query-driven model doesn’t handle it in an efficient way.

If you did rely on low-level manipulation of the graph database itself or being able to use traverse() in previous versions of Spring Data Neo4j, then your best options are:

Of course, there are pros and cons to both of these approaches, but these are largely outside the scope of this document. In general, for low-level, very high-performance operations like complex graph traversals you will get the best performance by writing a server-side extension. For most purposes, though, Cypher will be performant and expressive enough to perform the operations that you need.

Session allows you to save, load, loadAll and delete entities. The eagerness with which objects are retrieved is controlled by specifying the 'depth' argument to any of the load methods.

All of these basic CRUD methods just call onto the underlying methods of Session, albeit with transaction handling and exception translation managed for you by SDN’s TransactionManager bean.

The Session also allows execution of arbitrary Cypher queries via its query, queryForObject and queryForObjects methods. Cypher queries that return tabular results should be passed into the query method. An org.neo4j.ogm.session.result.Result is returned. This consists of org.neo4j.ogm.session.result.QueryStatistics representing statistics of modifying cypher statements if applicable, and an Iterable<Map<String,Object>> containing the raw data, of which nodes and relationships are mapped to domain entities if possible. The keys in each Map correspond to the names listed in the return clause of the executed Cypher query.

If you configured the Neo4jTransactionManager bean, any Session that is managed by Spring will automatically take part in thread contextual transactions. In order to do this you will need to wrap your service code using @Transactional or the TransactionTemplate.

For more details see Transactions

This chapter will point out the features for repository support for Neo4j and the Neo4j-OGM. This builds on the core repository support explained in Working with Spring Data Repositories. So make sure you have got a sound understanding of the basic concepts explained there.

The following table outlines the repositories functionality currently either supported, partially supported or not supported in SDN:

The Repository instances are created automatically through Spring and can be autowired into your Spring beans as required. You don’t have to provide an implementation for the interfaces you declared.

The recommended way of providing repositories is to define a repository interface per aggregate root and not for every domain class. The underlying Spring repository infrastructure will automatically detect these repositories, along with additional implementation classes, and create an injectable repository implementation to be used in services or other Spring beans.

The repositories provided by Spring Data Neo4j build on the composable repository infrastructure in Spring Data Commons. These allow for interface-based composition of repositories consisting of provided default implementations for certain interfaces and additional custom implementations for other methods.

Spring Data Neo4j comes with a single org.springframework.data.repository.PagingAndSortingRepository specialisation called Neo4jRepository<T. ID> used for all object-graph mapping repositories. This sub-interface also adds specific finder methods that take a depth argument to control the horizon with which related entities are fetched and saved. Generally, it provides all the desired repository methods. If other operations are required then the additional repository interfaces should be added to the individual interface declaration.

Neo4j is a transactional database, only allowing operations to be performed within transaction boundaries. Spring Data Neo4j integrates nicely with both the declarative transaction support with @Transactional as well as the manual transaction handling with TransactionTemplate.

The methods on the repositories instances are transactional by default. If you are, for example, simply just looking up an object through a repository then you do not need to define anything else: SDN will take of everything for you. That said, it is strongly recommended that you always annotate any service boundaries to the database with a @Transactional annotation. This way all your code for that method will always run in one transaction, even if you add a write operation later on.

More standard behaviour with transactions is using a facade or service implementation that typically covers more than one repository or database call as part of a 'Unit of Work'. Its purpose is to define transactional boundaries for non-CRUD operations:

This will cause all calls to addRoleToAllUsers(…) to run inside a transaction (participating in an existing one or create a new one if none already running). The transaction configuration at the repositories will be neglected as the outer transaction configuration determines the actual one used.

It is highly recommended that users understand how Spring transactions work. Below are some excellent resources:

This reference documentation is broken down into sections to help the user understand specifics of how Neo4j-OGM works.

Consult the version table to determine which version of Neo4j-OGM to use with a particular version of Neo4j and related technologies.

For building an application, your build automation tool needs to be configured to include the Neo4j-OGM dependencies.

Neo4j-OGM dependencies consist of neo4j-ogm-core, together with the relevant dependency declarations on the driver you want to use. Neo4j-OGM provides support for connecting to Neo4j by configuring one of the following Drivers:

Neo4j-OGM projects can be built using Maven, Gradle or any other build system that utilises Maven’s artifact repository structure.

There are several ways to supply configuration to Neo4j-OGM:

These methods are described below. They are also available as code in the examples.

For configuration through properties file or configuration builder the driver is automatically inferred from given URI. Empty URI means embedded driver with impermanent database.

Neo4j-OGM uses SLF4J to log statements. In production, you can set the log level in a file called logback.xml to be found at the root of the classpath. Please see the Logback manual for further details.

The @NodeEntity annotation is used to declare that a POJO class is an entity backed by a node in the graph database. Entities handled by Neo4j-OGM must have one empty public constructor to allow the library to construct the objects.

Fields on the entity are by default mapped to properties of the node. Fields referencing other node entities (or collections thereof) are linked with relationships.

@NodeEntity annotations are inherited from super-types and interfaces. It is not necessary to annotate your domain objects at every inheritance level.

Entity fields can be annotated with annotations like @Property, @Id, @GeneratedValue, @Transient or @Relationship. All annotations live in the org.neo4j.ogm.annotation package. Marking a field with the transient modifier has the same effect as annotating it with @Transient; it won’t be persisted to the graph database.

The default label is the simple class name of the annotated entity. There are some rules to determine if parent classes also contribute their label to the child class:

If the label (as you can see in the example above) or the value attribute of the @NodeEntity annotation is set it will replace the default label applied to the node in the database.

Saving a simple object graph containing one actor and one film using the above annotated objects would result in the following being persisted in Neo4j.

When annotating your objects, you can choose to NOT apply the annotations on the fields. OGM will then use conventions to determine property names in the database for each field.

In this case, a graph similar to the following would be persisted.

While this will map successfully to the database, it’s important to understand that the names of the properties and relationship types are tightly coupled to the class’s member names. Renaming any of these fields will cause parts of the graph to map incorrectly, hence the recommendation to use annotations.

Please read Non-annotated properties and best practices for more details and best practices on this.

Every field of an entity that references one or more other node entities is backed by relationships in the graph. These relationships are managed by Neo4j-OGM automatically.

The simplest kind of relationship is a single object reference pointing to another entity (1:1). In this case, the reference does not have to be annotated at all, although the annotation may be used to control the direction and type of the relationship. When setting the reference, a relationship is created when the entity is persisted. If the field is set to null, the relationship is removed.

It is also possible to have fields that reference a set of entities (1:N). Neo4j-OGM supports the following types of entity collections:

For graph to object mapping, the automatic transitive loading of related entities depends on the depth of the horizon specified on the call to Session.load(). The default depth of 1 implies that related node or relationship entities will be loaded and have their properties set, but none of their related entities will be populated.

If this Set of related entities is modified, the changes are reflected in the graph once the root object (Actor, in this case) is saved. Relationships are added, removed or updated according to the differences between the root object that was loaded and the corresponding one that was saved..

Neo4j-OGM ensures by default that there is only one relationship of a given type between any two given entities. The exception to this rule is when a relationship is specified as either OUTGOING or INCOMING between two entities of the same type. In this case, it is possible to have two relationships of the given type between the two entities, one relationship in either direction.

If you don’t care about the direction then you can specify direction=Relationship.UNDIRECTED which will guarantee that the path between two node entities is navigable from either side.

For example, consider the PARTNER relationship between two companies, where (A)-[:PARTNER_OF]→(B) implies (B)-[:PARTNER_OF]→(A). The direction of the relationship does not matter; only the fact that a PARTNER_OF relationship exists between these two companies is of importance. Hence an UNDIRECTED relationship is the correct choice, ensuring that there is only one relationship of this type between two partners and navigating between them from either entity is possible.

To access the full data model of graph relationships, POJOs can also be annotated with @RelationshipEntity, making them relationship entities. Just as node entities represent nodes in the graph, relationship entities represent relationships. Such POJOs allow you to access and manage properties on the underlying relationships in the graph.

Fields in relationship entities are similar to node entities, in that they’re persisted as properties on the relationship. For accessing the two endpoints of the relationship, two special annotations are available: @StartNode and @EndNode. A field annotated with one of these annotations will provide access to the corresponding endpoint, depending on the chosen annotation.

For controlling the relationship-type a String attribute called type is available on the @RelationshipEntity annotation. Like the simple strategy for labelling node entities, if this is not provided then the name of the class is used to derive the relationship type, although it’s converted into SNAKE_CASE to honour the naming conventions of Neo4j relationships. As of the current version of Neo4j-OGM, the type must be specified on the @RelationshipEntity annotation as well as its corresponding @Relationship annotations. This can also be done without naming the attribute but only providing the value.

Note that the Actor also contains a reference to a Role. This is important for persistence, even when saving the Role directly, because paths in the graph are written starting with nodes first and then relationships are created between them. Therefore, you need to structure your domain models so that relationship entities are reachable from node entities for this to work correctly.

Additionally, Neo4j-OGM will not persist a relationship entity that doesn’t have any properties defined. If you don’t want to include properties in your relationship entity then you should use a plain @Relationship instead. Multiple relationship entities which have the same property values and relate the same nodes are indistinguishable from each other and are represented as a single relationship by Neo4j-OGM.

Every node and relationship persisted to the graph must have an ID. Neo4j-OGM uses this to identify and re-connect the entity to the graph in memory. Identifier may be either a primary id or a native graph id (the technical id attributed by Neo4j at node creation time).

For primary id use the @Id on a field of any supported type or a field with provided AttributeConverter. A unique index is created for such property (if index creation is enabled). User code should either set the id manually when the entity instance is created or id generation strategy should be used. It is not possible to store an entity with null id value and no generation strategy.

For native graph id use @Id @GeneratedValue (with default strategy InternalIdStrategy). The field type must be Long. This id is assigned automatically upon saving the entity to the graph and user code should never assign a value to it.

An entity can be looked up by this either type of id by using Session.load(Class<T>, ID) and Session.loadAll(Class<T>, Collection<ID>) methods.

It is possible to have both natural and native id in one entity. In such situation lookups prefer the primary id.

If the field of type Long is simply named 'id' then it is not necessary to annotate it with @Id @GeneratedValue as Neo4j-OGM will use it automatically as native id.

Optimistic locking is supported by Neo4j-OGM to provide concurrency control. To use optimistic locking define a field annotated with @Version annotation. The field is then managed by Neo4j-OGM and used to perform optimistic locking checks when updating entities. The type of the field must be Long and an entity may contain only one such field.

Typical scenario where optimistic locking is used then looks like follows:

Optimistic locking check is performed for

When an optimistic locking failure happens following operations are performed on the Session:

As we touched on earlier, it is not necessary to annotate property fields as they are persisted by default. Fields that are annotated as @Transient or with transient are exempted from persistence. All fields that contain primitive values are persisted directly to the graph. All fields convertible to a String using the conversion services will be stored as a string. Neo4j-OGM includes default type converters for commonly used types, for a full list see Built-in type conversions.

Custom converters are also specified by using @Convert - this is discussed in detail later on.

Collections of primitive or convertible values are stored as well. They are converted to arrays of their type or strings respectively.

Node property names can be explicitly assigned by setting the name attribute. For example @Property(name="last_name") String lastName. The node property name defaults to the field name when not specified.

A method annotated with @PostLoad will be called once the entity is loaded from the database.

Neo4j-OGM supports mapping annotated and non-annotated objects models. It’s possible to save any POJO without annotations to the graph, as the framework applies conventions to decide what to do. This is useful in cases when you don’t have control over the classes that you want to persist. The recommended approach, however, is to use annotations wherever possible, since this gives greater control and means that code can be refactored safely without risking breaking changes to the labels and relationships in your graph.

Annotated and non-annotated objects can be used within the same project without issue.

The object graph mapping comes into play whenever an entity is constructed from a node or relationship. This could be done explicitly during the lookup or create operations of the Session but also implicitly while executing any graph operation that returns nodes or relationships and expecting mapped entities to be returned.

Entities handled by Neo4j-OGM must have one empty public constructor to allow the library to construct the objects.

Unless annotations are used to specify otherwise, the framework will attempt to map any of an object’s "simple" fields to node properties and any rich composite objects to related nodes. A "simple" field is any primitive, boxed primitive or String or arrays thereof, essentially anything that naturally fits into a Neo4j node property. For related entities the type of a relationship is inferred by the bean property name.

Indexes based on labels and properties are supported with the @Index annotation. Any property field annotated with @Index will have an appropriate schema index created. For @Index(unique=true) a constraint is created.

You may add as many indexes or constraints as you like to your class. A class outside any inheritance hierarchy leads to the creation of indexes for its annotated fields on its label. A class inheriting from a super-class and declaring an index on a field shadowing the same field of the super class leads to the creation of an index for that field on it’s own label. A non-abstract super-class will index that field on it’s corresponding label as well. In cases where there is no index definition in a sub-class at al, the first index-defining super class will define the index on its label in the database.

Composite indexes based on label and multiple properties are supported with @CompositeIndex annotation. The annotation is to be placed at the class level. All properties specified must exist within the class or one of its superclasses. It is possible to create multiple composite indexes by repeating the annotation.

Providing unique=true parameter will create a node key constraint instead of a composite index.

Existence constraints for a property is supported with @Required annotation. It is possible to annotate properties in both node entities and relationship entities. For node entities the label of declaring class is used to create the constraint. For relationship entities the relationship type is used - such type must be defined on leaf class.

By default index management is set to None.

If you would like Neo4j-OGM to manage your schema creation there are several ways to go about it.

Only classes marked with @Index, @CompositeIndex or @Required will be used. Indexes will always be generated with the containing class’s label and the annotated property’s name. An abstract class containing indexes or constraints must have @NodeEntity annotation present. Index generation behaviour can be defined in ogm.properties by defining a property called: indexes.auto and providing a value of:

Below is a table of all options available for configuring Auto-Indexing.

The SessionFactory is needed by Neo4j-OGM to create instances of Session as required. This also sets up the object-graph mapping metadata when constructed, which is then used across all Session objects that it creates. The packages to scan for domain object metadata should be provided to the SessionFactory constructor.

A Session is used to drive the object-graph mapping framework. It keeps track of the changes that have been made to entities and their relationships. The reason it does this is so that only entities and relationships that have changed get persisted on save, which is particularly efficient when working with large graphs. Once an entity is tracked by the session, reloading this entity within the scope of the same session will result in the session cache returning the previously loaded entity. However, the subgraph in the session will expand if the entity or its related entities retrieve additional relationships from the graph.

The lifetime of the Session can be managed in code. For example, associated with single fetch-update-save cycle or unit of work.

If your application relies on long-running sessions then you may not see changes made from other users and find yourself working with outdated objects. On the other hand, if your sessions have a too narrow scope then your save operations can be unnecessarily expensive, as updates will be made to all objects if the session isn’t aware of the those that were originally loaded.

There’s therefore a trade off between the two approaches. In general, the scope of a Session should correspond to a "unit of work" in your application.

If you want to fetch fresh data from the graph, then this can be achieved by using a new session or clearing the current sessions context using Session.clear(). This feature should be used with caution because it will clear the whole cache and it needs to get rebuild on the next operation. Also Neo4j-OGM won’t be able to do any dirty tracking between the operations that are separated by the Session.clear() call.

Basic operations are limited to CRUD operations on entities and executing arbitrary Cypher queries; more low-level manipulation of the graph database is not possible.

Given that the Neo4j-OGM framework is driven by Cypher queries alone, there’s no way to work directly with Node and Relationship objects in remote server mode.

If you find yourself in trouble because of the omission of these features, then your best option is to write a Cypher query to perform the operations on the nodes/relationships instead.

In general, for low-level, very high-performance operations like complex graph traversals you will get the best performance by writing a server-side extension. For most purposes, though, Cypher will be performant and expressive enough to perform the operations that you need.

Session allows to save, load, loadAll and delete entities with transaction handling and exception translation managed for you. The eagerness with which objects are retrieved is controlled by specifying the depth argument to any of the load methods.

Entity persistence is performed through the save() method on the underlying Session object.

Under the bonnet, the implementation of Session has access to the MappingContext that keeps track of the data that has been loaded from Neo4j during the lifetime of the session. Upon invocation of save() with an entity, it checks the given object graph for changes compared with the data that was loaded from the database. The differences are used to construct a Cypher query that persists the deltas to Neo4j before repopulating it’s state based on the response from the database server.

Neo4j-OGM doesn’t automatically commit when a transaction closes, so an explicit call to save(…​) is required in order to persist changes to the database.

Entities can be loaded from Neo4j-OGM through the use of the session.loadXXX() methods or via session.query()/session.queryForObject() which will accept your own Cypher queries (See section below on cypher queries).

Neo4j-OGM includes the concept of persistence horizon (depth). On any individual request, the persistence horizon indicates how many relationships should be traversed in the graph when loading or saving data. A horizon of zero means that only the root object’s properties will be loaded or saved, a horizon of 1 will include the root object and all its immediate neighbours, and so on. This attribute is enabled via a depth argument available on all session methods, but Neo4j-OGM chooses sensible defaults so that you don’t have to specify the depth attribute unless you want change the default values.

Neo4j-OGM will automatically perform the following type conversions:

java.time.Instant based dates are stored in the database using UTC.

Two dedicated annotations are provided to modify the date conversion:

They need to be applied to an attribute for a custom string format or in case you want to store a date or datetime value as long:

Alternatively, if you want to store java.util.Date or java.time.Instant as long values, use the @DateLong annotation:

Collections of primitive or convertible values are also automatically mapped by converting them to arrays of their type or strings respectively.

In order to define bespoke type conversions for particular members, you can annotate a field with @Convert. One of either two convert implementations can be used. For simple cases where a single property maps to a single field, with type conversion, specify an implementation of AttributeConverter.

You could then apply this to your class as follows:

When more than one node property is to be mapped to a single field, use: CompositeAttributeConverter.

And just as with an AttributeConverter, a CompositeAttributeConverter could be applied to your class as follows:

There are four types of events:

Events are fired for every @NodeEntity or @RelationshipEntity object that is created, updated or deleted, or otherwise affected by a save or delete request. This includes:

The events mechanism introduces two new interfaces, Event and EventListener.

The Event interface

The Event interface is implemented by PersistenceEvent. Whenever an application wishes to handle an event it will be given an instance of Event, which exposes the following methods:

The event listener interface

The EventListener interface provides methods allowing implementing classes to handle each of the different Event types:

There are two way to register an event listener:

In this example we register an anonymous EventListener to inject a UUID onto new objects before they’re saved

The EventListener above is fine, but we’ve had to create three methods for events we don’t intend to handle. It would be preferable if we didn’t have to do this each time we needed an EventListener.

The EventListenerAdapter is an abstract class providing a no-op implementation of the EventListener interface. If you don’t need to handle all the different types of persistence event you can create a subclass of EventListenerAdapter instead and override just the methods for the event types you’re interested in.

For example:

Something to bear in mind is that once an EventListener has been registered it will continue to respond to all persistence events. Sometimes you may want only to handle events for a short period of time, rather than for the duration of the entire session.

If you are done with an EventListener you can stop it from firing any more events by invoking session.dispose(…​), passing in the EventListener to be disposed of.

To remove an event listener across multiple sessions use the deregister method on the SessionFactory.

As mentioned previously, events are not only fired for the top-level objects being saved but for all their connected objects as well.

Connected objects are any objects reachable in the domain model from the top-level object being saved. Connected objects can be many levels deep in the domain model graph.

In this way, the events mechanism allows us to capture events for objects that we didn’t explicitly save ourselves.

When we delete a type, all the nodes with a label corresponding to that type are deleted in the graph. The affected objects are not enumerated by the events mechanism (they may not even be known). Instead, _DELETE events will be raised for the type:

When saving or deleting a collection of objects, separate events are fired for each object in the collection, rather than for the collection itself.

Events are partially ordered. PRE_ events are guaranteed to fire before any POST_ event within the same save or delete request. However, the internal ordering of the PRE_ events and POST_ events with the request is undefined.

The previous examples show how events fire when the underlying node representing an entity is updated or deleted in the graph. Events are also fired when a save or delete request results in the modification, addition or deletion of a relationship in the graph.

For example, if you delete a Document object that is contained in the documents collection of a Folder, events will be fired for the Document as well as the Folder, to reflect the fact that the relationship between the folder and the document has been removed in the graph.

The event mechanism guarantees to not fire more than one event of the same type for an object in a save or delete request.

When running unit tests, it can be useful to see what Neo4j-OGM is doing, and in particular to see the Cypher requests being transferred between your application and the database. Neo4j-OGM uses slf4j along with Logback as its logging framework and by default the log level for all Neo4j-OGM components is set to WARN, which does not include any Cypher output. To change Neo4j-OGM log level, create a file logback-test.xml in your test resources folder, configured as shown below:

To find out more about Causal Clustering architecture please see the reference.

Causal Clustering only works with the Neo4j Bolt Driver (1.1.0 onwards). Trying to set this up with the HTTP or Embedded Driver will not work. The Bolt driver will fully handle any load balancing, which operate in concert with the Causal Cluster to spread the workload. New cluster-aware sessions, managed on the client-side by the Bolt drivers, alleviate complex infrastructure concerns for developers.

A typical Neo4j HA cluster will consist of a primary node and a couple of secondary nodes for providing failover capability and optionally for handling reads. (Although it is possible to write to secondary nodes, this is uncommon because it requires additional effort to synchronise a secondary node with the primary node node.)

The <repositories /> element triggers the setup of the Spring Data repository infrastructure. The most important attribute is base-package, which defines the package to scan for Spring Data repository interfaces. See “XML Configuration”. The following table describes the attributes of the <repositories /> element:

The <populator /> element allows to populate the a data store via the Spring Data repository infrastructure.^[1]

The following table lists the subject keywords generally supported by the Spring Data repository query derivation mechanism to express the predicate. Consult the store-specific documentation for the exact list of supported keywords, because some keywords listed here might not be supported in a particular store.

The following table lists the predicate keywords generally supported by the Spring Data repository query derivation mechanism. However, consult the store-specific documentation for the exact list of supported keywords, because some keywords listed here might not be supported in a particular store.

In addition to filter predicates, the following list of modifiers is supported:

The following table lists the return types generally supported by Spring Data repositories. However, consult the store-specific documentation for the exact list of supported return types, because some types listed here might not be supported in a particular store.

Spring Data Neo4j 4.2 significantly reduces complexity of configuration for application developers. There is no longer a need to extend from Neo4jConfiguration or define a Session bean. Configuration for various types of applications are described here