Entities (Apollo Federation)
In a federated graph, an entity is an object type that can resolve its fields across multiple subgraphs.
Entities have a unique identifier, called a key, that is used to fetch them from
a subgraph. In Strawberry entities are defined by annotating a type with the
@strawberry.federation.type
decorator, like in example below:
You can also use the Key
directive manually, like so:
Resolving references
When a GraphQL operation references an entity across multiple services, the
Apollo Router will fetch the entity from the subgraph that defines it. To do
this, the subgraph needs to be able to resolve the entity by its key. This is
done by defining a class method called resolve_reference
on the entity type.
For example, if we have a Book
entity type, we can define a
resolve_reference
method like this:
Strawberry provides a default implementation of resolve_reference
that
instantiates the object type using the data coming from the key. This means that
you can omit the resolve_reference
method if you don’t need to fetch any
additional data for your object type, like in the example below:
In the example above we are creating an entity called Book
that has a title
field and a reviews_count
field. This entity will contribute to the Book
entity in the supergraph and it will provide the reviews_count
field.
When the Apollo Router fetches the Book
entity from this subgraph, it will
call the resolve_reference
method with the id
of the book and, as mentioned
above, Strawberry will instantiate the Book
type using the data coming from
the key.