Realm allows you to define explicit relationships between the types of objects in an App. A relationship is an object property that references another Realm object rather than one of the primitive data types. You define relationships by setting the type of an object property to another object type in the property schema.
Relationships are direct references to other objects in a realm, which means that you don't need bridge tables or explicit joins to define a relationship like you would in a relational database. Instead you can access related objects by reading and writing to the property that defines the relationship. Realm executes read operations lazily as they come in, so querying a relationship is just as performant as reading a regular property.
There are three primary types of relationships between objects:
You can define relationships, collections, and embedded objects in your object schema using the following types:
RealmObject
RealmList <? extends RealmObject>
Use annotations to indicate whether a given field represents a foreign key relationship or an embedded object relationship. For more information, see Relationship Annotations.
A to-one relationship means that an object is related in a specific way to no more than one other object. You define a to-one relationship for an object type in its object schema by specifying a property where the type is the related Realm object type.
Setting a relationship field to null removes the connection between objects, but Realm does not delete the referenced object unless that object is embedded.
A to-many relationship means that an object is related in a specific way to multiple objects. You can create a relationship between one object and any number of objects using a field of type RealmList<T> where T is a Realm object in your application:
An inverse relationship links an object back to any other objects that refer to it in a defined to-one or to-many relationship. Relationship definitions are unidirectional, so you must explicitly define a property in the object's model as an inverse relationship.
For example, the to-many relationship "User has many Tasks" does not automatically create the inverse relationship "Task belongs to User". If you don't specify the inverse relationship in the object model, you would need to run a separate query to look up the user that is assigned to a given task.
To define an inverse relationship, define a LinkingObjects property in your object model. The LinkingObjects definition specifies the object type and property name of the relationship that it inverts.
Realm automatically updates implicit relationships whenever an object is added or removed in the specified relationship. You cannot manually set the value of an inverse relationship property.
Fields annotated with @LinkingObjects must be:
marked final
of type RealmResults<T> where T is the type at the opposite end of the relationship
Since relationships are many-to-one or many-to-many, following inverse relationships can result in zero, one, or many objects.
Like any other RealmResults set, you can query an inverse relationship.
Realm objects use getters and setters to persist updated field values to your realms. Always use getters and setters for updates.
To set up a many-to-one or one-to-one relationship, create a field whose type is a Realm object in your application:
import io.realm.RealmObject;public class Frog extends RealmObject { private String name; private int age; private String species; private String owner; private Frog bestFriend; public Frog(String name, int age, String species, String owner, Frog bestFriend) { this.name = name; this.age = age; this.species = species; this.owner = owner; this.bestFriend = bestFriend; } public Frog(){} public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public String getSpecies() { return species; } public void setSpecies(String species) { this.species = species; } public String getOwner() { return owner; } public void setOwner(String owner) { this.owner = owner; } public Frog getBestFriend() { return bestFriend; } public void setBestFriend(Frog bestFriend) { this.bestFriend = bestFriend; }}import io.realm.RealmObjectopen class Frog : RealmObject { var name: String? = null var age = 0 var species: String? = null var owner: String? = null var bestFriend: Frog? = null constructor( name: String?, age: Int, species: String?, owner: String?, bestFriend: Frog? ) { this.name = name this.age = age this.species = species this.owner = owner this.bestFriend = bestFriend } constructor() {} }When you declare a to-one relationship in your object model, it must be an optional property. If you try to make a to-one relationship required, Realm throws an exception at runtime.
Each Frog references either zero Frog instances or one other Frog instance. Nothing prevents multiple Frog instances from referencing the same Frog as a best friend; the distinction between a many-to-one and a one-to-one relationship is up to your application.
import io.realm.RealmList;import io.realm.RealmObject;public class Frog extends RealmObject { private String name; private int age; private String species; private String owner; private RealmList<Frog> bestFriends; public Frog(String name, int age, String species, String owner, RealmList<Frog> bestFriends) { this.name = name; this.age = age; this.species = species; this.owner = owner; this.bestFriends = bestFriends; } public Frog(){} public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public String getSpecies() { return species; } public void setSpecies(String species) { this.species = species; } public String getOwner() { return owner; } public void setOwner(String owner) { this.owner = owner; } public RealmList<Frog> getBestFriends() { return bestFriends; } public void setBestFriends(RealmList<Frog> bestFriends) { this.bestFriends = bestFriends; }}import io.realm.RealmListimport io.realm.RealmObjectopen class Frog : RealmObject { var name: String? = null var age = 0 var species: String? = null var owner: String? = null var bestFriends: RealmList<Frog>? = null constructor( name: String?, age: Int, species: String?, owner: String?, bestFriends: RealmList<Frog>? ) { this.name = name this.age = age this.species = species this.owner = owner this.bestFriends = bestFriends } constructor() {} }RealmList s are containers of RealmObject s, but otherwise behave like a regular collection. You can use the same object in multiple RealmList s.
By default, Realm relationships are unidirectional. You can follow a link from one class to a referenced class, but not in the opposite direction. Consider the following class defining a Toad with a list of frogFriends:
import io.realm.RealmList;import io.realm.RealmObject;public class Toad extends RealmObject { private RealmList<Frog> frogFriends; public Toad(RealmList<Frog> frogFriends) { this.frogFriends = frogFriends; } public Toad() {} public RealmList<Frog> getFrogFriends() { return frogFriends; } public void setFrogFriends(RealmList<Frog> frogFriends) { this.frogFriends = frogFriends; }}import io.realm.RealmListimport io.realm.RealmObjectopen class Toad : RealmObject { var frogFriends: RealmList<Frog>? = null constructor(frogFriends: RealmList<Frog>?) { this.frogFriends = frogFriends } constructor() {}}You can provide a link in the opposite direction, from Frog to Toad, with the @LinkingObjects annotation on a final (in Java) or val (in Kotlin) field of type RealmResults<T>:
import io.realm.RealmObject;import io.realm.RealmResults;import io.realm.annotations.LinkingObjects;public class Frog extends RealmObject { private String name; private int age; private String species; private String owner; @LinkingObjects("frogFriends") private final RealmResults<Toad> toadFriends = null; public Frog(String name, int age, String species, String owner) { this.name = name; this.age = age; this.species = species; this.owner = owner; } public Frog(){} public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public String getSpecies() { return species; } public void setSpecies(String species) { this.species = species; } public String getOwner() { return owner; } public void setOwner(String owner) { this.owner = owner; }}import io.realm.RealmObjectimport io.realm.RealmResultsimport io.realm.annotations.LinkingObjectsopen class Frog : RealmObject { var name: String? = null var age = 0 var species: String? = null var owner: String? = null @LinkingObjects("frogFriends") private val toadFriends: RealmResults<Toad>? = null constructor(name: String?, age: Int, species: String?, owner: String?) { this.name = name this.age = age this.species = species this.owner = owner } constructor() {} }Inverse relationship fields must be marked final.
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