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:

import strawberry


@strawberry.federation.type(keys=["id"])
class Book:
    id: strawberry.ID
    title: str  

You can also use the Key directive manually, like so:

import strawberry

from strawberry.federation.schema_directives import Key


@strawberry.type(directives=[Key(fields="id")])
class Book:
    id: strawberry.ID
    title: str  

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:

import strawberry


@strawberry.federation.type(keys=["id"])
class Book:
    id: strawberry.ID
    title: str

    @classmethod
    def resolve_reference(cls, id: strawberry.ID) -> "Book":
        # here we could fetch the book from the database
        # or even from an API
        return Book(id=id, title="My Book")  

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:

import strawberry


@strawberry.federation.type(keys=["id"])
class Book:
    id: strawberry.ID
    reviews_count: int = strawberry.field(resolver=lambda: 3)  

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.