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Unwrapping Types

Unwrapped entities are used to design value objects in your Java domain model whose properties are flattened out into the parent’s MongoDB Document.

Unwrapped Types Mapping

Consider the following domain model where User.name is annotated with @Unwrapped. The @Unwrapped annotation signals that all properties of UserName should be flattened out into the user document that owns the name property.

Example 1. Sample Code of unwrapping objects

class User {

    @Id
    String userId;

    @Unwrapped(onEmpty = USE_NULL) (1)
    UserName name;
}

class UserName {

    String firstname;

    String lastname;

}

{
  "_id" : "1da2ba06-3ba7",
  "firstname" : "Emma",
  "lastname" : "Frost"
}

1	When loading the `name` property its value is set to `null` if both `firstname` and `lastname` are either `null` or not present. By using `onEmpty=USE_EMPTY` an empty `UserName`, with potential `null` value for its properties, will be created.

For less verbose embeddable type declarations use @Unwrapped.Nullable and @Unwrapped.Empty instead @Unwrapped(onEmpty = USE_NULL) and @Unwrapped(onEmpty = USE_EMPTY). Both annotations are meta-annotated with JSR-305 @javax.annotation.Nonnull to aid with nullability inspections.

It is possible to use complex types within an unwrapped object. However, those must not be, nor contain unwrapped fields themselves.

Unwrapped Types field names

A value object can be unwrapped multiple times by using the optional prefix attribute of the @Unwrapped annotation. By dosing so the chosen prefix is prepended to each property or @Field("…") name in the unwrapped object. Please note that values will overwrite each other if multiple properties render to the same field name.

Example 2. Sample Code of unwrapped object with name prefix

class User {

    @Id
    String userId;

    @Unwrapped.Nullable(prefix = "u_") (1)
    UserName name;

    @Unwrapped.Nullable(prefix = "a_") (2)
    UserName name;
}

class UserName {

    String firstname;

    String lastname;
}

{
  "_id" : "a6a805bd-f95f",
  "u_firstname" : "Jean",             (1)
  "u_lastname" : "Grey",
  "a_firstname" : "Something",        (2)
  "a_lastname" : "Else"
}

1	All properties of `UserName` are prefixed with `u_`.
2	All properties of `UserName` are prefixed with `a_`.

While combining the @Field annotation with @Unwrapped on the very same property does not make sense and therefore leads to an error. It is a totally valid approach to use @Field on any of the unwrapped types properties.

Example 3. Sample Code unwrapping objects with @Field annotation

public class User {

	@Id
    private String userId;

    @Unwrapped.Nullable(prefix = "u-") (1)
    UserName name;
}

public class UserName {

	@Field("first-name")              (2)
    private String firstname;

	@Field("last-name")
    private String lastname;
}

{
  "_id" : "2647f7b9-89da",
  "u-first-name" : "Barbara",         (2)
  "u-last-name" : "Gordon"
}

1	All properties of `UserName` are prefixed with `u-`.
2	Final field names are a result of concatenating `@Unwrapped(prefix)` and `@Field(name)`.

Query on Unwrapped Objects

Defining queries on unwrapped properties is possible on type- as well as field-level as the provided Criteria is matched against the domain type. Prefixes and potential custom field names will be considered when rendering the actual query. Use the property name of the unwrapped object to match against all contained fields as shown in the sample below.

Example 4. Query on unwrapped object

UserName userName = new UserName("Carol", "Danvers")
Query findByUserName = query(where("name").is(userName));
User user = template.findOne(findByUserName, User.class);

db.collection.find({
  "firstname" : "Carol",
  "lastname" : "Danvers"
})

It is also possible to address any field of the unwrapped object directly using its property name as shown in the snippet below.

Example 5. Query on field of unwrapped object

Query findByUserFirstName = query(where("name.firstname").is("Shuri"));
List<User> users = template.findAll(findByUserFirstName, User.class);

db.collection.find({
  "firstname" : "Shuri"
})

Sort by unwrapped field.

Fields of unwrapped objects can be used for sorting via their property path as shown in the sample below.

Example 6. Sort on unwrapped field

Query findByUserLastName = query(where("name.lastname").is("Romanoff"));
List<User> user = template.findAll(findByUserName.withSort(Sort.by("name.firstname")), User.class);

db.collection.find({
  "lastname" : "Romanoff"
}).sort({ "firstname" : 1 })

Though possible, using the unwrapped object itself as sort criteria includes all of its fields in unpredictable order and may result in inaccurate ordering.

Field projection on unwrapped objects

Fields of unwrapped objects can be subject for projection either as a whole or via single fields as shown in the samples below.

Example 7. Project on unwrapped object.

Query findByUserLastName = query(where("name.firstname").is("Gamora"));
findByUserLastName.fields().include("name");                             (1)
List<User> user = template.findAll(findByUserName, User.class);

db.collection.find({
  "lastname" : "Gamora"
},
{
  "firstname" : 1,
  "lastname" : 1
})

1	A field projection on an unwrapped object includes all of its properties.

Example 8. Project on a field of an unwrapped object.

Query findByUserLastName = query(where("name.lastname").is("Smoak"));
findByUserLastName.fields().include("name.firstname");                   (1)
List<User> user = template.findAll(findByUserName, User.class);

db.collection.find({
  "lastname" : "Smoak"
},
{
  "firstname" : 1
})

1	A field projection on an unwrapped object includes all of its properties.

Query By Example on unwrapped object.

Unwrapped objects can be used within an Example probe just as any other type. Please review the Query By Example section, to learn more about this feature.

Repository Queries on unwrapped objects.

The Repository abstraction allows deriving queries on fields of unwrapped objects as well as the entire object.

Example 9. Repository queries on unwrapped objects.

interface UserRepository extends CrudRepository<User, String> {

	List<User> findByName(UserName username);         (1)

	List<User> findByNameFirstname(String firstname); (2)
}

1	Matches against all fields of the unwrapped object.
2	Matches against the `firstname`.

Index creation for unwrapped objects is suspended even if the repository create-query-indexes namespace attribute is set to true.

Update on Unwrapped Objects

Unwrapped objects can be updated as any other object that is part of the domain model. The mapping layer takes care of flattening structures into their surroundings. It is possible to update single attributes of the unwrapped object as well as the entire value as shown in the examples below.

Example 10. Update a single field of an unwrapped object.

Update update = new Update().set("name.firstname", "Janet");
template.update(User.class).matching(where("id").is("Wasp"))
   .apply(update).first()

db.collection.update({
  "_id" : "Wasp"
},
{
  "$set" { "firstname" : "Janet" }
},
{ ... }
)

Example 11. Update an unwrapped object.

Update update = new Update().set("name", new Name("Janet", "van Dyne"));
template.update(User.class).matching(where("id").is("Wasp"))
   .apply(update).first()

db.collection.update({
  "_id" : "Wasp"
},
{
  "$set" {
    "firstname" : "Janet",
    "lastname" : "van Dyne",
  }
},
{ ... }
)

Aggregations on Unwrapped Objects

The Aggregation Framework will attempt to map unwrapped values of typed aggregations. Please make sure to work with the property path including the wrapper object when referencing one of its values. Other than that no special action is required.

Index on Unwrapped Objects

It is possible to attach the @Indexed annotation to properties of an unwrapped type just as it is done with regular objects. It is not possible to use @Indexed along with the @Unwrapped annotation on the owning property.

public class User {

	@Id
    private String userId;

    @Unwrapped(onEmpty = USE_NULL)
    UserName name;                    (1)

    // Invalid -> InvalidDataAccessApiUsageException
    @Indexed                          (2)
    @Unwrapped(onEmpty = USE_Empty)
    Address address;
}

public class UserName {

    private String firstname;

    @Indexed
    private String lastname;           (1)
}

1	Index created for `lastname` in `users` collection.
2	Invalid `@Indexed` usage along with `@Unwrapped`