Relationships and navigation properties

Relationships between your database tables are translated into navigation properties in your entities.  We'll look at relationships and navigation properties in more depth here.

See the next section for how to retrieve related entities using navigation properties.

One-to-many

The most common type of relationship is a one-to-many (or 1:M), sometimes termed parent->child or principal->dependent relationships.  Here, the child entity contains both a foreign key, sometimes nullable, and a scalar navigation property to its parent, while the parent entity contains a collection of child entities.  

For example, an Order contains a collection of OrderDetails - Order.OrderDetails  - while each OrderDetail contains a reference to its parent Order - OrderDetail.Order .    

In generated code, you'll see the parent with its child collection, a RelatedEntityList<T>:  

 While the child will contain a reference to its parent:

One-to-one

A one-to-one relationship might be a shared primary key association, in which the primary key acts as a foreign key, or a one-to-one foreign key association, where a unique constraint is used with the foreign key.  The relationship can be optional.  

You'll also use a one-to-one relationship with table splitting - when you map multiple entities to a single database table.  Table splitting allows you to lazily load less frequently used properties or those with large payloads, such as an image.

Each entity will contain a scalar navigation property to the related end.  

Many-to-many

A many-to-many relationship involves defining a third table (called a junction or join table), whose primary key is composed of the foreign keys from both related tables.  This join table is abstracted out of the object model, so that both ends of the relationship contain a collection of related entities.

In the NorthwindIB database, we see Employee and Territory joined by the unfortunately named EmployeeTerritoryNoPayload table.  The generated entities each contain a collection of related entities:

Working with many-to-many relationships differs from other relationship types since there is no foreign key defined within the model.  You will always add or remove entities from the RelatedEntityList to add and remove entries from the join table in the database.  Due to the lack of a defined foreign key in the object model, certain actions are not available or work differently than in other relationship types.  For example, cache-only navigation on a many-to-many navigation property is not supported.