GraphQL "Cannot return null for non-nullable" [duplicate]

As I am beginner into GraphQL, even I ran into this issue. After going through each file individually I found that I forgot to import into my resolvers

import User from './User';**import Post from './Post';**const resolvers = [User, **Posts**];

Maybe this will help!

user: {type: UserType,args: {   username: { type: new GraphQLNonNull(GraphQLString) }},resolve: function(parentValue, args) {  return User.find( args ).exec(); // User.find({username: 'some name'}).exec(); // will work as matches your mongoose schema}

Previously, in the args you are providing an an object with nested object username so,

args: {  // this won't match your mongoose schema field as it's nested object  username: {    name: 'username',    type: new GraphQLNonNull(GraphQLString)  }}

so when the user queries and provides args then your args would be { username: { name: 'abcd' } }

// args = {username: {name: 'abcd'}}

and resolve() is executing User.find({username: {name: 'abcd'}}).exec();

/* searching for username{} object, butyour mongoose schema is username: String */

which doesn't match your database fields, which will always return an empty array [],also which will not match your GraphQL field type, as it is GraphQLNonNull

after viewing the gist the problem is with rootquery

the problem is with rootquery

let RootQuery = new GraphQLObjectType({    name: 'RootQueryType',    fields: () => ({        users: { type:UserQueries.users, resolve: UserQueries.users }        user: { type: UserQueries.user, resolve: UserQueries.user }    })});