Object Models - .NET SDK

Object Schema

An object schema is a configuration object that defines the properties and relationships of a Realm object. Realm client applications define object schemas with the native class implementation in their respective language using the Object Schema.

Object schemas specify constraints on object properties such as the data type of each property and whether or not a property is required. Schemas can also define relationships between object types in a realm.

Every App has a App Services Schema composed of a list of object schemas for each type of object that the realms in that application may contain. 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.

Primary Key

A primary key is a property that uniquely identifies an object. You can create a primary key with any of the following types (or their nullable counterparts):

• ObjectId

• UUID

• string

• char

• byte

• short

• int

• long

You may define a primary key on a single property for an object type as part of the object schema. Realm Database automatically indexes primary key properties, which allows you to efficiently read and modify objects based on their primary key.

If an object type has a primary key, then all objects of that type must include the primary key property with a value that is unique among objects of the same type in a realm.

The following example demonstrates how to designate a primary key in an object schema:

public partial class Dog : IRealmObject
{
    [PrimaryKey]
    public string Name { get; set; }
    public int Age { get; set; }
    public Person Owner { get; set; }
}

Indexes

Indexes support the efficient execution of queries in Realm Database. Without indexes, Realm Database must perform a collection scan, i.e. scan every document in a collection, to select those documents that match a query. If an appropriate index exists for a query, Realm Database can use the index to limit the number of documents that it must inspect.

Indexes are special data structures that store a small portion of a realm's data in an easy to traverse form. The index stores the value of a specific property ordered by the value of the property. The ordering of the index entries supports efficient equality matches and range-based query operations.

Adding an index can speed up some queries at the cost of slightly slower write times and additional storage and memory overhead. Indexes require space in your realm file, so adding an index to a property will increase disk space consumed by your realm file. Each index entry is a minimum of 12 bytes.

You can index properties with the following types:

• bool

• byte

• short

• int

• long

• DateTimeOffset

• char

• string

• ObjectId

• UUID

To index a property, use the [Indexed] attribute:

public partial class Person : IRealmObject
{
    [Indexed]
    public string Name { get; set; }
    public IList<Dog> Dogs { get; }
}

Required and Optional Properties

In C#, value types are implicitly non-nullable, but can be made optional by appending ?. Reference types, such as string and byte[], are implicitly nullable and can be made required by annotating them with the [Required] attribute. Properties of types that inherit from IRealmObject or IEmbeddedObject are special and can always be null. Applying [Required] on such a property will result in a compilation error.

You can use the [Required] attribute as seen in the following example:

public partial class Person : IRealmObject
{
    [Required]
    public string Name { get; set; }
    public IList<Dog> Dogs { get; }
}

Default Field Values

You can use the built-in language features to assign a default value to a property. In C#, you can assign a default value in the property declaration.

public partial class Person : IRealmObject
{
    public string Name { get; set; } = "foo";
}

Ignore a Property

If you don't want to save a property in your model to a realm, you can ignore that property. A property is ignored by default if it is not autoimplemented or does not have a setter.

Ignore a property from a Realm object model with the [Ignored] attribute:

// Rather than store an Image in Realm,
// store the path to the Image...
public string ThumbnailPath { get; set; }
// ...and the Image itself can be
// in-memory when the app is running:
[Ignored]
public Image Thumbnail { get; set; }

Rename a Property

By default, Realm Database uses the name defined in the model class to represent properties internally. In some cases you might want to change this behavior:

• To make it easier to work across platforms, since naming conventions differ.

• To change a property name in .NET without forcing a migration.

Choosing an internal name that differs from the name used in model classes has the following implications:

• Migrations must use the internal name when creating classes and properties.

• Schema errors reported will use the internal name.

Use the [MapTo] attribute to rename a property:

public partial class Person : IRealmObject
{
    [MapTo("moniker")]
    public string Name { get; set; }
}

Rename a Class

By default, Realm Database uses the name defined in the model class to represent classes internally. In some cases you might want to change this behavior:

• To support multiple model classes with the same simple name in different namespaces.

• To make it easier to work across platforms, since naming conventions differ.

• To use a class name that is longer than the 57 character limit enforced by Realm Database.

• To change a class name in .NET without forcing a migration.

Use the [MapTo] attribute to rename a class:

[MapTo("Human")]
public partial class Person : IRealmObject
{
    public string Name { get; set; }
}

Custom Setters

Realm will not store a property with a custom setter. To use a custom setter, store the property value in a private property and then map that value to a public property with the custom setter. Realm will store the private property, while you modify its value via the public property. In the following code, the private email property is stored in the realm, but the public Email property, which provides validation, is not persisted:

// This property will be stored in the Realm
private string email { get; set; }
// Custom validation of the email property.
// This property is *not* stored in Realm.
public string Email
{
    get { return email; }
    set
    {
        if (!value.Contains("@")) throw new Exception("Invalid email address");
        email = value;
    }
}