Mutations

As opposed to queries, mutations in GraphQL represent operations that modify server-side data and/or cause side effects on the server. For example, you can have a mutation that creates a new instance in your application or a mutation that sends an email. Like in queries, they accept parameters and can return anything a regular field can, including new types and existing object types. This can be useful for fetching the new state of an object after an update.

Let's improve our books project from the Getting started tutorial and implement a mutation that is supposed to add a book:

import strawberry


# Reader, you can safely ignore Query in this example, it is required by
# strawberry.Schema so it is included here for completeness
@strawberry.type
class Query:
    @strawberry.field
    def hello() -> str:
        return "world"


@strawberry.type
class Mutation:
    @strawberry.mutation
    def add_book(self, title: str, author: str) -> Book:
        print(f"Adding {title} by {author}")

        return Book(title=title, author=author)


schema = strawberry.Schema(query=Query, mutation=Mutation)

Like queries, mutations are defined in a class that is then passed to the Schema function. Here we create an addBook mutation that accepts a title and an author and returns a Book type.

We would send the following GraphQL document to our server to execute the mutation:

mutation {
  addBook(title: "The Little Prince", author: "Antoine de Saint-Exupéry") {
    title
  }
}

The addBook mutation is a simplified example. In a real-world application mutations will often need to handle errors and communicate those errors back to the client. For example we might want to return an error if the book already exists.

You can checkout our documentation on dealing with errors to learn how to return a union of types from a mutation.

Mutations without returned data

It is also possible to write a mutation that doesn't return anything.

This is mapped to a Void GraphQL scalar, and always returns null

@strawberry.type
class Mutation:
    @strawberry.mutation
    def restart() -> None:
        print(f'Restarting the server')

type Mutation {
  restart: Void
}

The Input Mutation Extension

It is usually useful to use a pattern of defining a mutation that receives a single input type argument called input.

Strawberry provides a helper to create a mutation that automatically creates an input type for you, whose attributes are the same as the args in the resolver.

For example, suppose we want the mutation defined in the section above to be an input mutation. We can add the InputMutationExtension to the field like this:

from strawberry.field_extensions import InputMutationExtension


@strawberry.type
class Mutation:
    @strawberry.mutation(extensions=[InputMutationExtension()])
    def update_fruit_weight(
        self,
        info: Info,
        id: strawberry.ID,
        weight: Annotated[
            float,
            strawberry.argument(description="The fruit's new weight in grams"),
        ],
    ) -> Fruit:
        fruit = ...  # retrieve the fruit with the given ID
        fruit.weight = weight
        ...  # maybe save the fruit in the database
        return fruit

That would generate a schema like this:

input UpdateFruitWeightInput {
  id: ID!

  """
  The fruit's new weight in grams
  """
  weight: Float!
}

type Mutation {
  updateFruitWeight(input: UpdateFruitWeightInput!): Fruit!
}