Object Models & Schemas - Swift SDK

Property Attributes

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. Conversely, 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.

Primary Keys

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.

Indexes

You can create an index on a given property of your model. Indexes speed up some queries, but have a negative impact on insert and update operation speed. Indexes also consume additional space on disk to store the actual index information.

Relationships

Realm Database supports to-one, to-many, and inverse relationships. You can specify a relationship by adding a reference or list of references to the related Realm object as a property of your Realm object.

Inverse relationship properties contain the set of Realm objects that point to that object instance through a to-one or to-many relationship. Inverse relationships automatically update themselves with corresponding backlinks. You can find the same set of Realm objects with a manual query, but the inverse relationship field reduces boilerplate query code and capacity for error.

Build View Models with Realm

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 can pass through or transform some or all of your realm object's properties. You can query a class projection, or observe it for changes.

Class projection enables you to create model abstractions for use in your view models. Class projection also simplifies testing with Realm Database.

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)

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 and Results with a mixture of parent and subclass

Summary

• Realm objects are of a type defined as you would any other class.
• The Realm object's schema defines the fields of the object and which fields are optional, which fields have a default value, which fields are indexed, and which fields are ignored.
• You can define to-one, to-many, and inverse relationships in your schema. Inverse relationships automatically form backlinks.
• Realm does not currently support structs. When required, create "in-memory copies" of Realm objects.
• Model inheritance is possible but significantly limited.