Relationships - .NET SDK

To-One Relationship

A to-one relationship means that an object is related in a specific way to no more than one other object. You define a to-one relationship for an object type in its object schema by specifying a property where the type is the related Realm object type.

public partial class Person : IRealmObject
{
    [PrimaryKey]
    [MapTo("_id")]
    public ObjectId Id { get; set; }
    public string Name { get; set; }
    public DateTimeOffset Birthdate { get; set; }
    public Dog? Dog { get; set; }
}
public partial class Dog : IRealmObject
{
    [PrimaryKey]
    [MapTo("_id")]
    public ObjectId Id { get; set; }
    public string Name { get; set; }
    public int Age { get; set; }
    public string Breed { get; set; } = String.Empty;
}

To query a direct relationship, you can use LINQ syntax. See the following example for how to query a one-to-one relationship:

var fidosPerson = realm.All<Person>().FirstOrDefault(p => p.Dog == dog);

To-Many Relationship

A to-many relationship means that an object is related in a specific way to multiple objects. You define a to-many relationship for an object type by specifying a property where the type is an IList<T> of the related Realm object type. Define the IList<T> with only a getter. You do not need to initialize it in the constructor, as Realm will generate a collection instance the first time the property is accessed:

// To add items to the IList<T>:
var person = new Person();
person.Dogs.Add(new Dog
{
    Name = "Caleb",
    Age = 7,
    Breed = "mutt"
});

public partial class Person : IRealmObject
{
    [PrimaryKey]
    [MapTo("_id")]
    public ObjectId Id { get; set; }
    public string Name { get; set; }
    public DateTimeOffset Birthdate { get; set; }
    public IList<Dog> Dogs { get; }
}
public partial class Dog : IRealmObject
{
    [PrimaryKey]
    [MapTo("_id")]
    public ObjectId Id { get; set; }
    public string Name { get; set; }
    public int Age { get; set; }
    public string Breed { get; set; } = String.Empty;
}

var katieAndHerDogs = realm.All<Person>().
    Where(p => p.Name == "Katie")
    .FirstOrDefault();

Inverse Relationship

An inverse relationship links an object back to any other objects that refer to it in a defined to-one or to-many relationship. Relationship definitions are unidirectional, so you must explicitly define a property in the object's model as an inverse relationship.

For example, the to-many relationship "a User has many Items" does not automatically create the inverse relationship "an Item belongs to one User". If you don't specify the inverse relationship in the object model, you need to run a separate query to look up the user that is assigned to a given item.

To define the inverse relationship, define a getter-only IQueryable<T> property in your object model, where T is the source type of the relationship, and then annotate this property with a [Backlink(sourceProperty)] attribute, where "sourceProperty" is the name of the property on the other side of the relationship. The following example shows how to do this with the "User has many Items" scenario:

public partial class User : IRealmObject
{
    [PrimaryKey]
    [MapTo("_id")]
    public ObjectId Id { get; set; } = ObjectId.GenerateNewId();
    public string Name { get; set; }
    [Backlink(nameof(Item.Assignee))]
    public IQueryable<Item> Items { get; }
}
public partial class Item : IRealmObject
{
    [PrimaryKey]
    [MapTo("_id")]
    public ObjectId Id { get; set; } = ObjectId.GenerateNewId();
    public string Text { get; set; }
    public User? Assignee { get; set; }
}

In this example, note that:

• The Item object's Assignee property is a User object.

• The User object's Items property inverts the relationship and refers to all Item objects that contain this specific User in their Assignee property.

This, then, allows us to query the Item collection to get all Items assigned to a specific User.

To query the inverse relationship, you cannot use Linq. Instead, pass a string predicate. The following example shows how you could find all Users who have Items that contain the word "oscillator":

var oscillatorAssignees = realm.All<User>()
    .Filter("Items.Text CONTAINS 'oscillator'").ToList();
foreach (User u in oscillatorAssignees)
{
    Console.WriteLine(u.Name);
}