gqlHelpers.js

import {
    GraphQLObjectType, GraphQLList, GraphQLString, GraphQLInt, GraphQLBoolean,
  } from 'graphql'
import {attributeFields, resolver} from 'graphql-sequelize';
import db from '../../models'  

module.exports = modelName => modelRelationFunc => {
    
    const relationMethods = {
        listOf: (gqlRelationType, options={}) => ({
            [gqlRelationType.name]: {
                type: new GraphQLList(gqlRelationType),
                resolve: resolver(db[modelName][gqlRelationType.name], options)
            }
        }),
        single: (gqlRelationType, options={}, sequelizeRelationName) => {
            const relationName = sequelizeRelationName || gqlRelationType.name.slice(0, -1) 
            return{
                [relationName]: {
                    type: gqlRelationType,
                    resolve: resolver(db[modelName][relationName], options)
                }
            }
        }
    }
    return new GraphQLObjectType({
        name: modelName,
        fields: () => ({
            ...attributeFields(db[modelName], {}),
            ...modelRelationFunc
                ? modelRelationFunc(relationMethods).reduce((a,b) => ({...a, ...b}), {})
                : {}
        }),
    })
};