io.realm.annotations

Annotation Type LinkingObjects

@Retention(value=CLASS)
 @Target(value=FIELD)
public @interface LinkingObjects

Annotation for defining a reverse relationship from one class to another. This annotation can only be added to a field of the type RealmResults.

 To expose reverse relationships for use, create a declaration as follows:
 

 public class Person extends RealmObject {
   String name;
   Dog dog; // Normal relation
 }

 public class Dog extends RealmObject {
   // This holds all Person objects with a relation to this Dog object (= linking objects)
   \@LinkingObjects("dog")
   final RealmResults>Person< owners = null;
 }

 // Find all Dogs with at least one owner named John
 realm.where(Dog.class).equalTo("owners.name", "John").findAll();
 
 

In the above example `Person` is related to `Dog` through the field `dog`. This in turn means that an implicit reverse relationship exists between the class `Dog` and the class `Person`. This inverse relationship is made public and queryable by the `RealmResults` field annotated with `@LinkingObject`. This makes it possible to query properties of the dogs owner without having to manually maintain a "owner" field in the `Dog` class.

Linking objects have the following properties:

• The link is maintained by Realm and only works for managed objects.
• They can be queried just like normal relation.
• They can be followed just like normal relation.
• They are ignored when doing a `copyToRealm().`
• They are ignored when doing a `copyFromRealm().`
• They are ignored when using the various `createObjectFromJson*` and `createAllFromJson*` methods.
• Listeners on an object with a `@LinkingObject` field will not be triggered if the linking objects change, e.g: if another object drops a reference to this object.

In addition, they have the following restrictions:

• @Ignore takes precedence. A @LinkingObjects annotation on @Ignore field will be ignored.
• The annotated field cannot be @Required.
• The annotated field must be `final`.
• The annotation argument (the name of the backlinked field) is required.
• The annotation argument must be a simple field name. It cannot contain periods ('.').
• The annotated field must be of type `RealmResults>T<` where T is concrete class that extends `RealmModel`.

Note that when the source of the reverse reference (`dog` in the case above) is a `List`, there is a reverse reference for each forward reference, even if both forward references are to the same object. If the `Person` class above were defined as: public class DogLover extends RealmObject { String name; List<Dog> dogs = new ArrayList<Dog>; } then the following code executes without error Dog fido = new Dog(); DogLover john = new DogLover() john.dogs.add(fido); john.dogs.add(fido); assert john.dogs.size() == 2; assert fido.owners.size() == 2;

Querying inverse relationship is like querying any RealmResults. This means that an inverse relationship cannot be null but it can be empty (length is 0). It is possible to query fields in the source class. This is equivalent to link queries. Please read for more information.

Optional Element Summary

Optional Elements

Modifier and Type	Optional Element and Description
String	value The name of a field that contains a relation to an instance of the class containing this annotation.

Element Detail

value

public abstract String value

The name of a field that contains a relation to an instance of the class containing this annotation. If this argument is not provided the annotation processor will abort with an IllegalArgumentException.

Default:

""