CRUD - Create - Swift SDK

Write Transactions

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 handles writes in terms of transactions. A transaction is a list of read and write operations that Realm treats as a single indivisible 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.

All writes must happen in a transaction.

A realm allows only one open transaction at a time. Realm 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 either commits it or cancels it:

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

Run a Transaction

The Swift SDK 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 runs it, Realm cancels the transaction. Otherwise, Realm commits the transaction immediately after the callback.

About The Examples On This Page

The examples on this page use the following models:

Create an Object

Initialize Objects with a Value

You can initialize an object by passing an initializer value to Object.init(value:). The initializer value can be a key-value coding compliant object, a dictionary, or an array containing one element for each managed property.

You can even initialize related or embedded objects by nesting initializer values:

Create an Object with JSON

Realm does not directly support JSON, but you can use JSONSerialization.jsonObject(with:options:) to convert JSON into a value that you can pass to Realm.create(_:value:update:).

Nested objects or arrays in the JSON map to to-one or to-many relationships.

The JSON property names and types must match the destination object schema exactly. For example:

• float properties must be initialized with float-backed NSNumbers.
• Date and Data properties cannot be inferred from strings. Convert them to the appropriate type before passing to Realm.create(_:value:update:).
• Required properties cannot be null or missing in the JSON.

Realm ignores any properties in the JSON not defined in the object schema.

Create an Embedded Object

To create an embedded object, assign an instance of the embedded object to a parent object's property:

Create an Object with a Map Property

When you create an object that has a map property, you can set the values for keys in a few ways:

• Set keys and values on the object and then add the object to the realm
• Set the object's keys and values directly inside a write transaction
• Use key-value coding to set or update keys and values inside a write transaction

let realm = try! Realm()
// Record a dog's name and current city
let dog = Dog()
dog.name = "Wolfie"
dog.currentCity = "New York"
// Set map values
dog.favoriteParksByCity["New York"] = "Domino Park"
// Store the data in a realm
try! realm.write {
    realm.add(dog)
    // You can also set map values inside a write transaction
    dog.favoriteParksByCity["Chicago"] = "Wiggly Field"
    dog.favoriteParksByCity.setValue("Bush Park", forKey: "Ottawa")
}

Create an Object with a MutableSet Property

You can create objects that contain MutableSet properties as you would any Realm object, but you can only mutate a MutableSet within a write transaction. This means you can only set the value(s) of a mutable set property within a write transaction.

let realm = try! Realm()
// Record a dog's name and current city
let dog = Dog()
dog.name = "Maui"
dog.currentCity = "New York"
// Store the data in a realm. Add the dog's current city
// to the citiesVisited MutableSet
try! realm.write {
    realm.add(dog)
    // You can only mutate the MutableSet in a write transaction.
    // This means you can't set values at initialization, but must do it during a write.
    dog.citiesVisited.insert(dog.currentCity)
}
// You can also add multiple items to the set.
try! realm.write {
    dog.citiesVisited.insert(objectsIn: ["Boston", "Chicago"])
}
print("\(dog.name) has visited: \(dog.citiesVisited)")

Create an Object with an AnyRealmValue Property

When you create an object with an AnyRealmValue property, you must specify the type of the value you store in the property. The Realm Swift SDK provides an AnyRealmValue enum that iterates through all of the types the AnyRealmValue can store.

Later, when you read an AnyRealmValue, you must check the type before you do anything with the value.

// Create a Dog object and then set its properties
let myDog = Dog()
myDog.name = "Rex"
// This dog has no companion.
// You can set the field's type to "none", which represents `nil`
myDog.companion = .none
// Create another Dog whose companion is a cat.
// We don't have a Cat object, so we'll use a string to describe the companion.
let theirDog = Dog()
theirDog.name = "Wolfie"
theirDog.companion = .string("Fluffy the Cat")
// Another dog might have a dog as a companion.
// We do have an object that can represent that, so we can specify the
// type is a Dog object, and even set the object's value.
let anotherDog = Dog()
anotherDog.name = "Fido"
// Note: this sets Spot as a companion of Fido, but does not set
// Fido as a companion of Spot. Spot has no companion in this instance.
anotherDog.companion = .object(Dog(value: ["name": "Spot"]))
// Add the dogs to the realm
let realm = try! Realm()
try! realm.write {
    realm.add([myDog, theirDog, anotherDog])
}
// After adding these dogs to the realm, we now have 4 dog objects.
let dogs = realm.objects(Dog.self)
XCTAssertEqual(dogs.count, 4)