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Relationships - iOS SDK

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  • Key Concepts
  • To-One Relationship
  • To-Many Relationship
  • Inverse Relationship
  • Summary

MongoDB Realm allows you to define explicit relationships between the types of objects in a Realm app. A relationship is an object property that references another Realm object rather than one of the primitive data types. You define relationships by setting the type of an object property to another object type in the property schema.

Relationships are direct references to other objects in a realm, which means that you don't need bridge tables or explicit joins to define a relationship like you would in a relational database. Instead you can access related objects by reading and writing to the property that defines the relationship. Realm Database executes read operations lazily as they come in, so querying a relationship is just as performant as reading a regular property.

There are three primary types of relationships between objects:

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.

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

Tip

To learn how to define a to-one relationship, see Define a To-One Relationship Property.

A to-many relationship means that an object is related in a specific way to multiple objects. A to-many relationship in Realm Database is a list property containing references to other objects.

Tip

To learn how to define a to-many relationship, see Define a To-Many Relationship Property.

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 "User has many Tasks" does not automatically create the inverse relationship "Task belongs to User". If you don't specify the inverse relationship in the object model, you would need to run a separate query to look up the user that is assigned to a given task.

Realm Database automatically updates implicit relationships whenever an object is added or removed in the specified relationship. You cannot manually set the value of an inverse relationship property.

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

Tip

To learn how to define an inverse relationship, see Define an Inverse Relationship Property.

  • A relationship is an object property that allows an object to reference other objects of the same or another object type.
  • Relationships are direct references. You can access related objects directly through a relationship property without writing any type of join.
  • Realm Database supports to-one, to-many, and inverse relationships.
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On this page

  • Key Concepts
  • To-One Relationship
  • To-Many Relationship
  • Inverse Relationship
  • Summary