多对多graphql模式错误

Question

I'm new with GrpahQL and I'm trying to simulate Many to Many relationship between Users and Groups. I have the followinf types defined in my schema:

// UserType.js
const {
    GraphQLObjectType,
    GraphQLString,
    GraphQLList,
    GraphQLID } = require('graphql');

const {
    GraphQLEmail } = require('graphql-custom-types');

const GroupType = require('./GroupType'); const AuthService = require('../../services/AuthService');

let authService = new AuthService();

const UserType = new GraphQLObjectType({
    name: "UserType",
    fields: () => ({
        id: { type: GraphQLID },
        user: { type: GraphQLString },
        password: { type: GraphQLString },
        name: { type: GraphQLString },
        lastname: { type: GraphQLString },
        email: { type: GraphQLEmail },
        groups: {
            type: new GraphQLList(GroupType),
            resolve(parentValue) {
                return authService.userGroups(userId);
            }
        }
    }) });


module.exports = UserType;

and this is the other file:

// GroupType.js
const {
    GraphQLObjectType,
    GraphQLString,
    GraphQLID,
    GraphQLList
} = require('graphql');

const UserType = require('./UserType');
const AuthService = require('../../services/AuthService');

let authService = new AuthService();


const GroupType = new GraphQLObjectType({
    name: "GroupType",
    fields: () => ({
        id: { type: GraphQLID },
        name: { type: GraphQLString },
        description: { type: GraphQLString },
        users: {
            type: new GraphQLList(UserType),
            resolve(parentArgs) {
                return authService.userGroups(parentArgs.id);
            }
        }
    })
});

module.exports = GroupType;

This example doesn't work for me because for some reason i got this error:

Error: Can only create List of a GraphQLType but got: [object Object].

This error happens only for the GroupType and not for the UserType when both are similars. What's going on here? What am I doing wrong?

Answer 1

The problem is that UserType requires GroupType , and GroupType requires UserType : this is known as a circular dependency.

What happens is that UserType.js gets required, exports an {} while finishing to run (this is standard Node.js module execution), requires GroupType , which requires back UserType and gets an empty object back, and exports the correct GraphQL GroupType to UserType . So UserType works because it is a list of GroupType , but GroupType doesn't it got an empty object for its require of UserType.

To circumvent this, you can use a runtime require in GroupType.js :

// GroupType.js
...

// Remove the line which requires UserType at the top
// const UserType = require('./UserType');
const AuthService = require('../../services/AuthService');

...

const GroupType = new GraphQLObjectType({
    ...
    fields: () => ({
        ...
        users: {
            type: new GraphQLList(require('./UserType')), // Require UserType at runtime
            ...
        }
    })
});

...