Accessing parent’s data in resolvers

It is quite common to want to be able to access the data from the field’s parent in a resolver. For example let’s say that we want to define a fullName field on our User . This would be our code:

import strawberry


@strawberry.type
class User:
    first_name: str
    last_name: str
    full_name: str  

type User {
  firstName: String!
  lastName: String!
  fullName: String!
}  

In this case full_name will need to access the first_name and last_name fields, and depending on whether we define the resolver as a function or as a method, we’ll have a few options! Let’s start with the defining a resolver as a function.

Accessing parent’s data in function resolvers

import strawberry


def get_full_name() -> str: ...


@strawberry.type
class User:
    first_name: str
    last_name: str
    full_name: str = strawberry.field(resolver=get_full_name)  

Our resolver is a function with no arguments, in order to tell Strawberry to pass us the parent of the field, we need to add a new argument with type strawberry.Parent[ParentType] , like so:

def get_full_name(parent: strawberry.Parent[User]) -> str:
    return f"{parent.first_name} {parent.last_name}"  

strawberry.Parent tells Strawberry to pass the parent value of the field, in this case it would be the User .

Note: strawberry.Parent accepts a type argument, which will then be used by your type checker to check your code!

Using root

Historically Strawberry only supported passing the parent value by adding a parameter called root :

def get_full_name(root: User) -> str:
    return f"{root.first_name} {root.last_name}"  

This is still supported, but we recommend using strawberry.Parent , since it follows Strawberry’s philosophy of using type annotations. Also, with strawberry.Parent your argument can have any name, for example this will still work:

def get_full_name(user: strawberry.Parent[User]) -> str:
    return f"{user.first_name} {user.last_name}"  

Accessing parent’s data in a method resolver

Both options also work when defining a method resolver, so we can still use strawberry.Parent in a resolver defined as a method:

import strawberry


@strawberry.type
class User:
    first_name: str
    last_name: str

    @strawberry.field
    def full_name(self, parent: strawberry.Parent[User]) -> str:
        return f"{parent.first_name} {parent.last_name}"  

But, here’s where things get more interesting. If this was a pure Python class, we would use self directly, right? Turns out that Strawberry also supports this!

Let’s update our resolver:

import strawberry


@strawberry.type
class User:
    first_name: str
    last_name: str

    @strawberry.field
    def full_name(self) -> str:
        return f"{self.first_name} {self.last_name}"  

Much better, no? self on resolver methods is pretty convenient, and it works like it should in Python, but there might be cases where it doesn’t properly follow Python’s semantics. This is because under the hood resolvers are actually called as if they were static methods by Strawberry.

Let’s see a simplified version of what happens when you request the full_name field, to do that we also need a field that allows to fetch a user:

import strawberry


@strawberry.type
class Query:
    @strawberry.field
    def user(self) -> User:
        return User(first_name="Albert", last_name="Heijn")  

When we do a query like this:

{
  user {
    fullName
  }
}  

We are pretty much asking to call the user function on the Query class, and then call the full_name function on the User class, similar to this code:

user = Query().user()

full_name = user.full_name()  

While this might work for this case, it won’t work in other cases, like when returning a different type, for example when fetching the user from a database:

import strawberry


@strawberry.type
class Query:
    @strawberry.field
    def user(self) -> User:
        # let's assume UserModel fetches data from the db and it
        # also has `first_name` and `last_name`
        user = UserModel.objects.first()

        return user  

In this case our pseudo code would break, since UserModel doesn’t have a full_name function! But it does work when using Strawberry (provided that the UserModel has both first_name and last_name fields).

As mentioned, this is because Strawberry class the resolvers as if they were plain functions (not bound to the class), similar to this:

# note, we are not instantiating the Query any more!
user = Query.user()  # note: this is a `UserModel` now

full_name = User.full_name(user)  

You’re probably thinking of staticmethod s and that’s pretty much what we are dealing with now! If you want to keep the resolver as a method on your class but also want to remove some of the magic around self , you can use the @staticmethod decorator in combination with strawberry.Parent :

import strawberry


@strawberry.type
class User:
    first_name: str
    last_name: str

    @strawberry.field
    @staticmethod
    def full_name(parent: strawberry.Parent[User]) -> str:
        return f"{parent.first_name} {parent.last_name}"  

Combining @staticmethod with strawberry.Parent is a good way to make sure that your code is clear and that you are aware of what’s happening under the hood, and it will keep your linters and type checkers happy!