This version is still in development and is not considered stable yet. For the latest stable version, please use Spring Data Neo4j 7.3.2! |
Query by Example
Introduction
This chapter provides an introduction to Query by Example and explains how to use it.
Query by Example (QBE) is a user-friendly querying technique with a simple interface. It allows dynamic query creation and does not require you to write queries that contain field names. In fact, Query by Example does not require you to write queries by using store-specific query languages at all.
This chapter explains the core concepts of Query by Example. The information is pulled from the Spring Data Commons module. Depending on your database, String matching support can be limited. |
Usage
The Query by Example API consists of four parts:
-
Probe: The actual example of a domain object with populated fields.
-
ExampleMatcher
: TheExampleMatcher
carries details on how to match particular fields. It can be reused across multiple Examples. -
Example
: AnExample
consists of the probe and theExampleMatcher
. It is used to create the query. -
FetchableFluentQuery
: AFetchableFluentQuery
offers a fluent API, that allows further customization of a query derived from anExample
. Using the fluent API lets you specify ordering projection and result processing for your query.
Query by Example is well suited for several use cases:
-
Querying your data store with a set of static or dynamic constraints.
-
Frequent refactoring of the domain objects without worrying about breaking existing queries.
-
Working independently of the underlying data store API.
Query by Example also has several limitations:
-
No support for nested or grouped property constraints, such as
firstname = ?0 or (firstname = ?1 and lastname = ?2)
. -
Store-specific support on string matching. Depending on your databases, String matching can support starts/contains/ends/regex for strings.
-
Exact matching for other property types.
Before getting started with Query by Example, you need to have a domain object. To get started, create an interface for your repository, as shown in the following example:
public class Person {
@Id
private String id;
private String firstname;
private String lastname;
private Address address;
// … getters and setters omitted
}
The preceding example shows a simple domain object.
You can use it to create an Example
.
By default, fields having null
values are ignored, and strings are matched by using the store specific defaults.
Inclusion of properties into a Query by Example criteria is based on nullability.
Properties using primitive types (int , double , …) are always included unless the ExampleMatcher ignores the property path.
|
Examples can be built by either using the of
factory method or by using ExampleMatcher
. Example
is immutable.
The following listing shows a simple Example:
Person person = new Person(); (1)
person.setFirstname("Dave"); (2)
Example<Person> example = Example.of(person); (3)
1 | Create a new instance of the domain object. |
2 | Set the properties to query. |
3 | Create the Example . |
You can run the example queries by using repositories.
To do so, let your repository interface extend QueryByExampleExecutor<T>
.
The following listing shows an excerpt from the QueryByExampleExecutor
interface:
QueryByExampleExecutor
public interface QueryByExampleExecutor<T> {
<S extends T> S findOne(Example<S> example);
<S extends T> Iterable<S> findAll(Example<S> example);
// … more functionality omitted.
}
Example Matchers
Examples are not limited to default settings.
You can specify your own defaults for string matching, null handling, and property-specific settings by using the ExampleMatcher
, as shown in the following example:
Person person = new Person(); (1)
person.setFirstname("Dave"); (2)
ExampleMatcher matcher = ExampleMatcher.matching() (3)
.withIgnorePaths("lastname") (4)
.withIncludeNullValues() (5)
.withStringMatcher(StringMatcher.ENDING); (6)
Example<Person> example = Example.of(person, matcher); (7)
1 | Create a new instance of the domain object. |
2 | Set properties. |
3 | Create an ExampleMatcher to expect all values to match.
It is usable at this stage even without further configuration. |
4 | Construct a new ExampleMatcher to ignore the lastname property path. |
5 | Construct a new ExampleMatcher to ignore the lastname property path and to include null values. |
6 | Construct a new ExampleMatcher to ignore the lastname property path, to include null values, and to perform suffix string matching. |
7 | Create a new Example based on the domain object and the configured ExampleMatcher . |
By default, the ExampleMatcher
expects all values set on the probe to match.
If you want to get results matching any of the predicates defined implicitly, use ExampleMatcher.matchingAny()
.
You can specify behavior for individual properties (such as "firstname" and "lastname" or, for nested properties, "address.city"). You can tune it with matching options and case sensitivity, as shown in the following example:
ExampleMatcher matcher = ExampleMatcher.matching()
.withMatcher("firstname", endsWith())
.withMatcher("lastname", startsWith().ignoreCase());
}
Another way to configure matcher options is to use lambdas (introduced in Java 8). This approach creates a callback that asks the implementor to modify the matcher. You need not return the matcher, because configuration options are held within the matcher instance. The following example shows a matcher that uses lambdas:
ExampleMatcher matcher = ExampleMatcher.matching()
.withMatcher("firstname", match -> match.endsWith())
.withMatcher("firstname", match -> match.startsWith());
}
Queries created by Example
use a merged view of the configuration.
Default matching settings can be set at the ExampleMatcher
level, while individual settings can be applied to particular property paths.
Settings that are set on ExampleMatcher
are inherited by property path settings unless they are defined explicitly.
Settings on a property patch have higher precedence than default settings.
The following table describes the scope of the various ExampleMatcher
settings:
Setting | Scope |
---|---|
Null-handling |
|
String matching |
|
Ignoring properties |
Property path |
Case sensitivity |
|
Value transformation |
Property path |
Fluent API
QueryByExampleExecutor
offers one more method, which we did not mention so far: <S extends T, R> R findBy(Example<S> example, Function<FluentQuery.FetchableFluentQuery<S>, R> queryFunction)
.
As with other methods, it executes a query derived from an Example
.
However, with the second argument, you can control aspects of that execution that you cannot dynamically control otherwise.
You do so by invoking the various methods of the FetchableFluentQuery
in the second argument.
sortBy
lets you specify an ordering for your result.
as
lets you specify the type to which you want the result to be transformed.
project
limits the queried attributes.
first
, firstValue
, one
, oneValue
, all
, page
, stream
, count
, and exists
define what kind of result you get and how the query behaves when more than the expected number of results are available.
Optional<Person> match = repository.findBy(example,
q -> q
.sortBy(Sort.by("lastname").descending())
.first()
);