Use DataLoaders in your Python GraphQL servers that have to run in a sync context (i.e. Django).

• Python 3.8+
• graphql-core >=3.2.0

This package can be installed from PyPi by running:

pip install graphql-sync-dataloaders

When creating your Strawberry Schema pass DeferredExecutionContext as the execution_context_class argument:

# schema.py
import strawberry
from graphql_sync_dataloaders import DeferredExecutionContext

schema = strawberry.Schema(Query, execution_context_class=DeferredExecutionContext)

Then create your dataloaders using the SyncDataLoader class:

from typing import List

from graphql_sync_dataloaders import SyncDataLoader

from .app import models  # your Django models

def load_users(keys: List[int]) -> List[User]:
    qs = models.User.objects.filter(id__in=keys)
    user_map = {user.id: user for user in qs}
    return [user_map.get(key, None) for key in keys]

user_loader = SyncDataLoader(load_users)

You can then use the loader in your resolvers and it will automatically be batched to reduce the number of SQL queries:

import strawberry

@strawberry.type
class Query:
    @strawberry.field
    def get_user(self, id: strawberry.ID) -> User:
        return user_loader.load(id)

Note: You probably want to setup your loaders in context. See https://strawberry.rocks/docs/guides/dataloaders#usage-with-context for more details

The following query will only make 1 SQL query:

fragment UserDetails on User {
  username
}

query {
  user1: getUser(id: '1') {
    ...UserDetails
  }
  user2: getUser(id: '2') {
    ...UserDetails
  }
  user3: getUser(id: '3') {
    ...UserDetails
  }
}

Requires graphene-django >=3.0.0b8

When setting up your GraphQLView pass DeferredExecutionContext as the execution_context_class argument:

# urls.py
from django.urls import path
from graphene_django.views import GraphQLView
from graphql_sync_dataloaders import DeferredExecutionContext

from .schema import schema

urlpatterns = [
    path(
        "graphql",
        csrf_exempt(
            GraphQLView.as_view(
                schema=schema, 
                execution_context_class=DeferredExecutionContext
            )
        ),
    ),
]

Then create your dataloaders using the SyncDataLoader class:

from typing import List

from graphql_sync_dataloaders import SyncDataLoader

from .app import models  # your Django models

def load_users(keys: List[int]) -> List[User]:
    qs = models.User.objects.filter(id__in=keys)
    user_map = {user.id: user for user in qs}
    return [user_map.get(key, None) for key in keys]

user_loader = SyncDataLoader(load_users)

You can then use the loader in your resolvers and it will automatically be batched to reduce the number of SQL queries:

import graphene

class Query(graphene.ObjectType):
    get_user = graphene.Field(User, id=graphene.ID)

    def resolve_get_user(root, info, id):
        return user_loader.load(id)

The following query will only make 1 SQL query:

fragment UserDetails on User {
  username
}

query {
  user1: getUser(id: '1') {
    ...UserDetails
  }
  user2: getUser(id: '2') {
    ...UserDetails
  }
  user3: getUser(id: '3') {
    ...UserDetails
  }
}

The SyncDataLoader.load function returns a SyncFuture object which, similar to a JavaScript Promise, allows you to chain results together using the then(on_success: Callable) function.

For example:

def get_user_name(userId: str) -> str:
    return user_loader.load(userId).then(lambda user: user["name"])

You can also chain together multiple DataLoader calls:

def get_best_friend_name(userId: str) -> str:
    return (
        user_loader.load(userId)
        .then(lambda user: user_loader.load(user["best_friend"]))
        .then(lambda best_friend: best_friend["name"])
      )

This library implements a custom version of the graphql-core ExecutionContext class that is aware of the SyncFuture objects defined in this library. A SyncFuture represents a value that hasn't been resolved to a value yet (similiar to asycnio Futures or JavaScript Promises) and that is what the SyncDataLoader returns when you call the .load function.

When the custom ExecutionContext encounters a SyncFuture that gets returned from a resolver and it keeps track of them. Then after the first pass of the exection it triggers the SyncFuture callbacks until there are none left. Once there are none left the data is fully resolved and can be returned to the caller synchronously. This allows us to implement a DataLoader pattern that batches calls to a loader function, and it allows us to do this in a fully synchronously way.

@Cito for graphql-core and for implementing the first version of this in graphql-python/graphql-core#155