Define a Realm Object Schema - .NET SDK¶

Many-to-One¶

To set up a many-to-one or one-to-one relationship, create a property in your application whose type inherits RealmObject or EmbeddedObject:

public class Dog : RealmObject
{
    // ... other property declarations
    public Person Owner { get; set; }
}
public class Person : RealmObject
{
    // ... other property declarations
    public string Name { get; set; }
}

Each Dog references zero or one Person instances. Nothing prevents multiple Dog instances from referencing the same Person as an owner; the distinction between a many-to-one and a one-to-one relationship is up to your application.

Setting a relationship property to null removes the connection between objects, but Realm Database does not delete the referenced object unless that object is embedded.

Many-to-Many¶

You can create a relationship between one object and any number of objects using a property of type IList<T> in your application, where T is a subclass of RealmObject or EmbeddedObject:

public class Dog : RealmObject
{
    // ... other property declarations
    public string Name { get; set; }
}
public class Person : RealmObject
{
    // ... other property declarations
    public IList<Dog> Dogs { get; }
}

Inverse Relationships¶

By default, Realm Database relationships are unidirectional. You can follow a link from one class to a referenced class, but not in the opposite direction. You can provide a link in the opposite direction with the [Backlink] attribute:

class Dog : RealmObject
{
    // To-one relationship from the Dog to its owner
    public Person Owner { get; set; }
}
class Person : RealmObject
{
    // An inverse relationship that returns all Dog instances that have Dog.Owner set to
    // the current Person.
    [Backlink(nameof(Dog.Owner))]
    public IQueryable<Dog> Dogs { get; }
    // To-many relationship, containing a collection of all hobbies the current person enjoys
    public IList<Hobby> Hobbies { get; }
}
class Hobby : RealmObject
{
    // An inverse relationship that returns all Person instances that have the current Hobby
    // instance in their Hobbies list.
    [Backlink(nameof(Person.Hobbies))]
    public IQueryable<Person> PeopleWithThatHobby { get; }
}

Since relationships are many-to-one or many-to-many, following inverse relationships can result in zero, one, or many objects.

Like any other RealmResults set, you can query an inverse relationship.

Primary Key¶

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

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 RealmObject or EmbeddedObject 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 class Person : RealmObject
{
    [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 class Person : RealmObject
{
    public string Name { get; set; } = "foo";
}

Index a Property¶

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

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

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 class Person : RealmObject
{
    [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 class Person : RealmObject
{
    public string Name { get; set; }
}