Object Models - Swift SDK
On this page
- Key Concept: Object Types & Schemas
- Realm Schema
- Model Inheritance
- Swift Structs
- Key Concept: Properties
- Key Concept: View Models with Realm
- Define a New Object Type
- Declare Properties
- Persisted Property Attributes
- Objective-C Dynamic Property Attributes
- Specify an Optional/Required Property
- Specify a Primary Key
- Index a Property
- Ignore a Property
- Declare Enum Properties
- Define a Class Projection
- About These Examples
- How to Define a Class Projection
Key Concept: Object Types & Schemas
Realm Database applications model data as objects composed of field-value pairs that each contain one or more supported data types.
Realm objects are regular Swift or Objective-C classes, but they also bring a few additional features like live queries. The Swift SDK memory maps Realm objects directly to native Swift or Objective-C objects, which means there's no need to use a special data access library, such as an ORM. Instead, you can work with Realm objects as you would any other class instance.
Every Realm object conforms to a specific object type, which is essentially a class that defines the properties and relationships for objects of that type. Realm guarantees that all objects in a realm conform to the schema for their object type and validates objects whenever they're created, modified, or deleted.
The following schema defines a Dog
object type with a string name,
optional string breed, date of birth, and primary key ID.
Realm Schema
A realm schema is a list of valid object schemas that a realm may contain. Every Realm object must conform to an object type that's included in its realm's schema.
By default, the Swift SDK automatically adds all classes in your project that derive from RLMObject or RLMEmbeddedObject to the realm schema.
To control which classes Realm adds to a realm schema, see Provide a Subset of Classes to a Realm.
If a realm already contains data when you open it, Realm Database validates each object to ensure that an object schema was provided for its type and that it meets all of the constraints specified in the schema.
For code examples that show how to configure and open a realm in the Swift SDK, see Configure & Open a Realm - Swift SDK.
Model Inheritance
You can subclass Realm Database models to share behavior between classes, but there are limitations. In particular, Realm does not allow you to:
- Cast between polymorphic classes: subclass to subclass, subclass to parent, parent to subclass
- Query on multiple classes simultaneously: for example, "get all instances of parent class and subclass"
- Multi-class containers:
List
andResults
with a mixture of parent and subclass
Check out the code samples for working around these limitations.
New in version 10.10.0: While you can't mix @Persisted
and @objc dynamic
property declarations
within a class, you can mix the notation styles across base and subclasses.
For example, a base class could have a @Persisted var foo: Int
property,
and a subclass could have an @objc dynamic var bar = 0
property, with
both persisted. However, the @objc dynamic
property would be ignored if
the @Persisted
property were within the same base or subclass.
Swift Structs
Realm Database does not support Swift structs as models for a variety of reasons. Realm's design focuses on “live” objects. This concept is not compatible with value type structs. By design, Realm provides features that are incompatible with these semantics, such as:
- Live data
- Reactive APIs
- Low memory footprint of data
- Good operation performance
- Lazy and cheap access to partial data
- Lack of data serialization/deserialization
- Keeping potentially complex object graphs synchronized
That said, it is sometimes useful to detach objects from their backing realm. This typically isn't an ideal design decision. Instead, developers use this as a workaround for temporary limitations in our library.
You can use key-value coding to initialize an unmanaged object as a copy of a managed object. Then, you can work with that unmanaged object like any other NSObject.
let standaloneModelObject = MyModel(value: persistedModelObject)
Key Concept: Properties
Your Realm object model is a collection of properties. On the most basic level, when you create your model, your declarations give Realm information about each property:
- The data type and whether the property is optional or required
- Whether Realm should store or ignore the property
- Whether the property is a primary key or should be indexed
Properties are also the mechanism for establishing relationships between Realm object types.
Key Concept: View Models with Realm
New in version 10.21.0.
You can work with a subset of your Realm Database object's properties by creating a class projection. A class projection is a class that passes through or transforms some or all of your Realm Database object's properties. Class projection enables you to build view models that use an abstraction of your object model. This simplifies using and testing Realm Database objects in your application.
With class projection, you can use a subset of your object's properties directly in the UI or transform them. When you use a class projection for this, you get all the benefits of Realm Database's live objects:
- The class-projected object live updates
- You can observe it for changes
- You can apply changes directly to the properties in write transactions
Define a New Object Type
Class names are limited to a maximum of 57 UTF-8 characters.
Declare Properties
When you declare the property attributes of a class, you can specify whether or not those properties should be managed by the realm. Managed properties are stored or updated in the database. Ignored properties are not stored to the database. You can mix managed and ignored properties within a class.
The syntax to mark properties as managed or ignored varies depending on which version of the SDK you use.
Persisted Property Attributes
New in version 10.10.0: The @Persisted
declaration style replaces the @objc dynamic
,
RealmOptional
, and RealmProperty
declaration notations from older
versions of the SDK. For an older version of the SDK, see:
Objective-C Dynamic Property Attributes.
Declare model properties that you want to store to the database as
@Persisted
. This enables them to access the underlying database data.
When you declare any properties as @Persisted
within a class, the other
properties within that class are automatically ignored.
If you mix @Persisted
and @objc dynamic
property declarations within
a class definition, any property attributes marked as @objc dynamic
will
be ignored.
Our Supported Property Types page contains a property declaration cheatsheet.
Objective-C Dynamic Property Attributes
Changed in version 10.10.0: This property declaration information is for versions of the SDK before 10.10.0.
Declare dynamic App Services model properties in the Objective-C runtime. This enables them to access the underlying database data.
You can either:
- Use
@objc dynamic var
to declare individual properties - Use
@objcMembers
to declare a class. Then, declare individual properties withdynamic var
.
Use let
to declare LinkingObjects
, List
, RealmOptional
and
RealmProperty
. The Objective-C runtime cannot represent these
generic properties.
Changed in version 10.8.0: RealmProperty
replaces RealmOptional
Our Supported Property Types page contains a property declaration cheatsheet.
For reference on which types Realm Database supports for use as properties, see Supported Property Types.
Property names are limited to a maximum of 63 UTF-8 characters.
Specify an Optional/Required Property
Specify a Primary Key
You can designate a property as the primary key of your class.
Primary keys allow you to efficiently find, update, and upsert objects.
Primary keys are subject to the following limitations:
- You can define only one primary key per object model.
- Primary key values must be unique across all instances of an object in a realm. Realm Database throws an error if you try to insert a duplicate primary key value.
- Primary key values are immutable. To change the primary key value of an object, you must delete the original object and insert a new object with a different primary key value.
- Embedded objects cannot define a primary key.
Index a Property
You can create an index on a given property of your model. Indexes speed up queries using equality and IN operators. They make insert and update operation speed slightly slower. Indexes take up more space in the realm file. It's best to only add indexes when optimizing the read performance for specific situations.
Realm supports indexing for string, integer, boolean, Date
, UUID
,
ObjectId
, and AnyRealmValue
properties.
New in version 10.8.0: UUID
and AnyRealmValue
types
Ignore a Property
Ignored properties behave exactly like normal properties. They can't be used in queries and won't trigger Realm notifications. You can still observe them using KVO.
Realm automatically ignores read-only properties.
Declare Enum Properties
Define a Class Projection
About These Examples
The examples in this section use a simple data set. The two Realm object
types are Person
and an embedded object Address
. A Person
has
a first and last name, an optional Address
, and a list of friends
consisting of other Person
objects. An Address
has a city and country.
See the schema for these two classes, Person
and Address
, below:
class Person: Object { var firstName = "" var lastName = "" var address: Address? var friends = List<Person>() } class Address: EmbeddedObject { var city: String = "" var country = "" }
How to Define a Class Projection
New in version 10.21.0.
Define a class projection by creating a class of type Projection. Specify the Object
or EmbeddedObject base whose
properties you want to use in the class projection. Use the @Projected
property wrapper to declare a property that you want to project from a
@Persisted
property on the base object.
When you use a List or a MutableSet in a class projection, the type in the class projection should be ProjectedCollection.
class PersonProjection: Projection<Person> { Person.firstName) var firstName // Passthrough from original object (\ Person.address?.city) var homeCity // Rename and access embedded object property through keypath (\ Person.friends.projectTo.firstName) var firstFriendsName: ProjectedCollection<String> // Collection mapping (\}
When you define a class projection, you can transform the original @Persisted
property in several ways:
- Passthrough: the property is the same name and type as the original object
- Rename: the property has the same type as the original object, but a different name
- Keypath resolution: use keypath resolution to access properties of the original object, including embedded object properties
- Collection mapping: Project lists or
mutable sets of
Object
s orEmbeddedObject
s as a collection of primitive values - Exclusion: when you use a class projection, the underlying object's
properties that are not
@Projected
through the class projection are excluded. This enables you to watch for changes to a class projection and not see changes for properties that are not part of the class projection.