CRUD - Delete - Java SDK

About the Examples on this Page

The examples on this page use the data model of a project management app that has two Realm object types: Project and Task. A Project has zero or more Tasks.

See the schema for these two classes, Project and Task, below:

Java

java


import org.bson.types.ObjectId;

import io.realm.RealmObject;
import io.realm.annotations.PrimaryKey;
import io.realm.annotations.RealmClass;
import io.realm.annotations.Required;

public class ProjectTask extends RealmObject {
    @PrimaryKey
    public ObjectId _id;
    @Required
    public String name;
    public String assignee;
    public int progressMinutes;
    public boolean isComplete;
    public int priority;
    @Required
    public String _partition;
}

java


import org.bson.types.ObjectId;

import io.realm.RealmList;
import io.realm.RealmObject;
import io.realm.annotations.PrimaryKey;
import io.realm.annotations.RealmClass;
import io.realm.annotations.Required;

public class Project extends RealmObject {
    @PrimaryKey
    public ObjectId _id;
    @Required
    public String name;
    public RealmList<ProjectTask> tasks = new RealmList<>();
}

Kotlin

kotlin

import io.realm.RealmObject
import io.realm.annotations.PrimaryKey
import io.realm.annotations.Required
import org.bson.types.ObjectId

open class ProjectTask(
    @PrimaryKey
    var _id: ObjectId = ObjectId(),
    @Required
    var name: String = "",
    var assignee: String? = null,
    var progressMinutes: Int = 0,
    var isComplete: Boolean = false,
    var priority: Int = 0,
    var _partition: String = ""
): RealmObject()

kotlin

import io.realm.RealmList
import io.realm.RealmObject
import io.realm.annotations.PrimaryKey
import io.realm.annotations.Required
import org.bson.types.ObjectId

open class Project(
    @PrimaryKey
    var _id: ObjectId = ObjectId(),
    @Required
    var name: String = "",
    var tasks: RealmList<ProjectTask> = RealmList(),
): RealmObject()

Delete an Object

To delete an object from a realm, use either the dynamic or static versions of the deleteFromRealm() method of a RealmObject subclass.

The following example shows how to delete one object from its realm with deleteFromRealm():

Java

java

realm.executeTransaction(r -> {
    // Get a turtle named "Tony".
    Turtle tony = r.where(Turtle.class).equalTo("name", "Tony").findFirst();
    tony.deleteFromRealm();
    // discard the reference
    tony = null;
});

Kotlin

kotlin

realm.executeTransaction { r: Realm ->
    // Get a turtle named "Tony".
    var tony = r.where(Turtle::class.java)
        .equalTo("name", "Tony")
        .findFirst()
    tony!!.deleteFromRealm()
    // discard the reference
    tony = null
}

Tip: The SDK throws an error if you try to use an object after it has been deleted.

Delete Multiple Objects

To delete an object from a realm, use the deleteAllFromRealm() method of the RealmResults instance that contains the objects you would like to delete. You can filter the RealmResults down to a subset of objects using the where() method.

The following example demonstrates how to delete a collection from a realm with deleteAllFromRealm():

Java

java

realm.executeTransaction(r -> {
    // Find turtles older than 2 years old.
    RealmResults<Turtle> oldTurtles = r.where(Turtle.class).greaterThan("age", 2).findAll();
    oldTurtles.deleteAllFromRealm();
});

Kotlin

kotlin

realm.executeTransaction { r: Realm ->
    // Find turtles older than 2 years old.
    val oldTurtles = r.where(Turtle::class.java)
        .greaterThan("age", 2)
        .findAll()
    oldTurtles.deleteAllFromRealm()
}

Delete an Object and its Dependent Objects

Sometimes, you have dependent objects that you want to delete when you delete the parent object. We call this a chaining delete. Realm does not delete the dependent objects for you. If you do not delete the objects yourself, they will remain orphaned in your realm. Whether or not this is a problem depends on your application's needs.

Currently, the best way to delete dependent objects is to iterate through the dependencies and delete them before deleting the parent object.

The following example demonstrates how to perform a chaining delete by first deleting all of Ali's turtles, then deleting Ali:

Java

java

realm.executeTransaction(r -> {
    // Find a turtle enthusiast named "Ali"
    TurtleEnthusiast ali = r.where(TurtleEnthusiast.class).equalTo("name", "Ali").findFirst();
    // Delete all of ali's turtles
    ali.getTurtles().deleteAllFromRealm();
    ali.deleteFromRealm();
});

Kotlin

kotlin

realm.executeTransaction { r: Realm ->
    // Find a turtle enthusiast named "Ali"
    val ali = r.where(TurtleEnthusiast::class.java)
        .equalTo("name", "Ali").findFirst()
    // Delete all of ali's turtles
    ali!!.turtles!!.deleteAllFromRealm()
    ali.deleteFromRealm()
}

Delete All Objects of a Specific Type

Realm supports deleting all instances of a Realm type from a realm.

The following example demonstrates how to delete all Turtle instances from a realm with delete():

Java

java

realm.executeTransaction(r -> {
    r.delete(Turtle.class);
});

Kotlin

kotlin

realm.executeTransaction { r: Realm ->
    r.delete(Turtle::class.java)
}

Delete All Objects in a Realm

It is possible to delete all objects from the realm. This does not affect the schema of the realm. This is useful for quickly clearing out your realm while prototyping.

The following example demonstrates how to delete everything from a realm with deleteAll():

Java

java

realm.executeTransaction(r -> {
    r.deleteAll();
});

Kotlin

kotlin

realm.executeTransaction { r: Realm ->
    r.deleteAll()
}

Delete an Object Using an Iterator

Because realm collections always reflect the latest state, they can appear, disappear, or change while you iterate over a collection. To get a stable collection you can iterate over, you can create a snapshot of a collection's data. A snapshot guarantees the order of elements will not change, even if an element is deleted.

For an example, refer to Iteration.