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Writes

Overview

Realm Database uses a highly efficient storage engine to persist objects. You can create objects in a realm, update objects in a realm, and eventually delete objects from a realm. Because these operations modify the state of the realm, we call them writes.

Realm Database handles writes in terms of transactions. A transaction is a list of read and write operations that Realm Database treats as a single indivisable operation. In other words, a transaction is all or nothing: either all of the operations in the transaction succeed or none of the operations in the transaction take effect.

Remember

All writes must happen in a transaction.

A realm allows only one open transaction at a time. Realm Database blocks other writes on other threads until the open transaction is complete. Consequently, there is no race condition when reading values from the realm within a transaction.

When you are done with your transaction, Realm Database either commits it or cancels it:

  • When Realm Database commits a transaction, Realm Database writes all changes to disk. For synced realms, Realm Database queues the change for synchronization with MongoDB Realm.
  • When Realm Database cancels a write transaction or an operation in the transaction causes an error, all changes are discarded (or “rolled back”).

Realm is ACID Compliant

Realm Database guarantees that transactions are ACID compliant. This means that all committed write operations are guaranteed to be valid and that clients don’t see transient states in the event of a system crash.

  • Realm Database establishes atomicity by grouping operations in transactions and rolling back all operations in a transaction if any of them fail.
  • Realm Database establishes consistency and avoids data corruption by validating changes against the schema. If the result of any write operation is not valid, Realm Database cancels and rolls back the entire transaction.
  • Realm Database establishes isolation by allowing only one writer at a time. This ensures thread safety between transactions.
  • Finally, Realm Database establishes durability by writing to disk immediately when a transaction is committed. In the event of an app crash, for example, the changes are not lost or corrupted.

Run a Transaction

Realm Database represents each transaction as a callback function that contains zero or more read and write operations. To run a transaction, define a transaction callback and pass it to the realm’s write method. Within this callback, you are free to create, read, update, and delete on the realm. If the code in the callback throws an exception when Realm Database runs it, Realm Database cancels the transaction. Otherwise, Realm Database commits the transaction immediately after the callback.

Concurrency Concerns

Since transactions block each other, it is best to avoid opening transactions on both the UI thread and a background thread. If you are using Sync, avoid opening transactions on the UI thread altogether, as Realm Database processes synchronizations on a background thread. If a background transaction blocks your UI thread’s transaction, your app may appear unresponsive.

Example

The following code shows how to run a transaction with the realm’s write method. If the code in the callback throws an exception, Realm Database cancels the transaction. Otherwise, Realm Database commits the transaction.

realm.write(() => {
  // Create someone to take care of some dogs.
  const ali = realm.create("Person", { id: 1, name: "Ali" });

  // Find dogs younger than 2.
  const puppies = realm.objects("Dog").filtered("age < 2");

  // Loop through to update.
  puppies.forEach(puppy => {
    // Give all puppies to Ali.
    puppy.owner = ali;
  });
});

Create an Object

In general, instantiate Realm objects as you would any other object. In a transaction, you can add the object to the realm if the realm’s schema includes the object type. When you add an instance to the realm, it becomes managed by that realm.

With the Java and JavaScript SDKs, instead use the realm’s factory method in a transaction to instantiate your class. This automatically inserts the instance into the realm.

Example

This code demonstrates how to create an object with Realm Database:

// Declare the instance.
let dog;

// Open a transaction.
realm.write(() => {
  // Assign a newly-created instance to the variable.
  dog = realm.create("Dog", { name: "Max", age: 5 });
});

Upsert an Object

An upsert is a write operation that either inserts a new object with a given primary key or updates an existing object that already has that primary key. We call this an upsert because it is an “update or insert” operation. This is useful when an object may or may not already exist, such as when bulk importing a dataset into an existing realm. Upserting is an elegant way to update existing entries while adding any new entries.

Example

This code demonstrates how to upsert an object with realm. We create a new user named “Drew” and then update their name to “Andy” via upsert:

realm.write(() => {
  // Add a new person to the realm. Since nobody with ID 1234
  // has been added yet, this adds the instance to the realm.
  const drew = realm.create("Person", { id: 1234, name: "Drew" }, "modified");

  // Judging by the ID, it's the same person, just with a different name.

  // If an object exists, setting the third parameter (`updateMode`) to
  // "modified" only updates properties that have changed, resulting in
  // faster operations.
  const andy = realm.create("Person", { id: 1234, name: "Andy" }, "modified");
});

Update an Object

Within a transaction, you can update a Realm object the same way you would update any other object in your language of choice: just assign a new value to the property or update the property.

Example

This code changes the dog’s name to “Wolfie” and increments the age by 1:

// Open a transaction.
realm.write(() => {
  // Get a dog to update.
  const dog = realm.objects("Dog")[0];

  // Update some properties on the instance.
  // These changes are saved to the realm.
  dog.name = "Wolfie";
  dog.age += 1;
});

Update a Collection

Realm Database supports collection-wide updates. A collection update applies the same update to specific properties of several objects in a collection at once.

Example

The following code demonstrates how to update a collection. Thanks to the implicit inverse relationship between the Dog’s owner property and the Person’s dogs property, Realm Database automatically updates Ali’s list of dogs.

realm.write(() => {
  // Create someone to take care of some dogs.
  const ali = realm.create("Person", { id: 1, name: "Ali" });

  // Find dogs younger than 2.
  const puppies = realm.objects("Dog").filtered("age < 2");

  // Loop through to update.
  puppies.forEach(puppy => {
    // Give all puppies to Ali.
    puppy.owner = ali;
  });
});

Delete an Object

In general, to delete an object from a realm, pass the instance to the delete method of the realm within a transaction.

With the Java SDK, call deleteFromRealm() on the instance itself.

Do not use objects after delete

Realm Database throws an error if you try to use an object after it has been deleted.

Example

The following code shows how to delete one object from its realm:

realm.write(() => {
  // Delete the dog from the realm.
  realm.delete(dog);

  // Discard the reference.
  dog = null;
});

Delete a Collection

In general, to delete a collection of objects from a realm, pass the collection to the delete method of the realm within a transaction.

With the Java SDK, call deleteFromRealm() on the collection itself.

Example

The following code demonstrates how to delete a collection from a realm:

realm.write(() => {
  // Find dogs younger than 2 years old.
  const puppies = realm.objects("Dog").filtered("age < 2");

  // Delete the collection from the realm.
  realm.delete(puppies);
});

Cascading Delete

Sometimes, you have dependent objects that you want to delete when you delete the parent object. We call this a cascading delete. Realm Database will 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.

Example

The following code demonstrates how to perform a cascading delete by first deleting all of Ali’s dogs, then deleting Ali:

realm.write(() => {
  // Delete all of Ali's dogs.
  realm.delete(ali.dogs);
  // Delete Ali.
  realm.delete(ali);
});

Delete All Instances of a Type

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

Example

The following code demonstrates how to delete all Dog instances from a realm:

realm.write(() => {
  // Delete all instances of Dog from the realm.
  realm.deleteModel("Dog");
});

Delete Everything

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.

Example

The following code demonstrates how to delete everything from a realm:

realm.write(() => {
  // Delete all objects from the realm.
  realm.deleteAll();
});

Summary

  • Realm Database handles writes in terms of transactions. All writes must occur in a transaction.
  • Realm Database transactions are ACID compliant.
  • To write to Realm Database, define the transaction in a callback function that you pass to the realm’s write method.
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