Return custom error message

Hi there!

I’d like to know the better way to raise an exception on a Mutation. For example, while trying to create a User with an email address that already exists.

if email
    user = User.objects.filter(email=email).first()

    if user:
       # ?

return CreateUser(username=username, password=password)

I know that I can raise Exception('message'), but is this the best way? Can we do it differently?

Thanks!

[ariannedee]

I usually return a GraphQLError.

from graphql import GraphQLError
...
raise GraphQLError('That email already exists')

It takes a few more arguments: nodes, stack, source, and positions, but I have never used these.

[jkimbo]

I’m going to close this issue since it’s stale. However just to add: I’ve found that modeling expected errors as part of your mutation response is essential. Your client needs to know what to do if your mutation fails. To do that I’ve found that modeling your response types as unions works really well since it allows you explicitly model the errors you’re expecting. So in a createUser mutation similar to yours @jonatasbaldin you can do this:

class CreateUserFailUsernameExists(graphene.ObjectType):
	suggested_alternatives = graphene.List(graphene.String)
	error_message = graphene.String(required=True)


class CreateUserFailOther(graphene.ObjectType):
	error_message = graphene.String(required=True)


class CreateUserSuccess(graphene.ObjectType):
	user = graphene.Field(User, required=True)


class CreateUserPayload(graphene.Union):
    class Meta:
        types = (CreateUserFailUsernameExists, CreateUserFailOther, CreateUserSuccess)


class CreateUser(graphene.Mutation):
	class Arguments:
		username = graphene.String(required=True)
		password = graphene.String(required=True)
	
	Output = CreateUserPayload

	def mutate(root, info, username, password):
    	if User.objects.filter(username=username).exists():
			return CreateUserFailUsernameExists(
				error_message="Username already exists",
				suggested_alternatives=get_alternatives(username)
			)
		try:
			user = create_user(username, password)
			return CreateUserSuccess(user=user)
		except:
			return CreateUserFailOther(error_message="Something went wrong")

Then in your client your mutation query becomes:

mutation createUser($username, String!, $password: String!) {
	createUser(username: $username, password: $password) {
		__typename
		... on CreateUserFailUsernameExists {
			suggestedAlternatives
		}
		... on CreateUserFailOther {
			errorMessage
		}
		... on CreateUserSuccess {
			user {
				id
			}
		}
	}
}

Then you just need to check the __typename value to figure out if there was an error (and what kind) or if the mutation succeed.