title: Graphql Unknown argument on field
body: <p>I am new to GraphQL. Whenever I try to send args on (posts) child node like the query below, I  get an error msg &quot;Unknown argument id on field posts of type user&quot;. I want to bring a particular post only, not all of them.</p>
  <pre><code>{ people(id:[1,2]) {
              id
              username
              posts(id:2) {
                title
                tags {
                  name
                }
              }
            }
          }
  </code></pre>
  <p>Here is my Schema.js file ..</p>
  <pre><code>var graphql = require('graphql');
  var Db = require('./db');
  var users  = new graphql.GraphQLObjectType({
    name : 'user',
    description : 'this is user info',
    fields : function(){
      return {
        id :{
          type : graphql.GraphQLInt,
          resolve(user){
            return user.id;
          }
        },
        username :{
          type : graphql.GraphQLString,
          resolve(user){
            return user.username;
          }
        },
  
        posts:{
          id:{
            type : graphql.GraphQLString,
            resolve(post){
              return post.id;
            }
          },
          type: new  graphql.GraphQLList(posts),
          resolve(user){
            return user.getPosts();
          }
        }
  
  
      }
    }
  });
  
  
  
  var posts  = new graphql.GraphQLObjectType({
    name : 'Posts',
    description : 'this is post info',
    fields : function(){
      return {
        id :{
          type : graphql.GraphQLInt,
          resolve(post){
            return post.id;
          }
        },
        title :{
          type : graphql.GraphQLString,
          resolve(post){
            return post.title;
          }
        },
        content:{
          type : graphql.GraphQLString,
          resolve(post){
            return post.content;
          }
        },
        person :{
          type: users,
          resolve(post){
            return post.getUser();
          }
        },
  
        tags :{
          type: new  graphql.GraphQLList(tags),
          resolve(post){
            return post.getTags();
          }
        }
      }
    }
  });
  
  var tags  = new graphql.GraphQLObjectType({
    name : 'Tags',
    description : 'this is Tags info',
    fields : function(){
      return {
        id :{
          type : graphql.GraphQLInt,
          resolve(tag){
            return tag.id;
          }
        },
        name:{
          type : graphql.GraphQLString,
          resolve(tag){
            return tag.name;
          }
        },
        posts :{
          type: new  graphql.GraphQLList(posts),
          resolve(tag){
            return tag.getPosts();
          }
        }
      }
    }
  });
  
  var query = new graphql.GraphQLObjectType({
    name : 'query',
    description : 'Root query',
    fields : function(){
      return {
       people :{
          type : new  graphql.GraphQLList(users),
          args :{
            id:{type: new graphql.GraphQLList(graphql.GraphQLInt)},
            username:{
              type: graphql.GraphQLString
            }
          },
          resolve(root,args){
            return Db.models.user.findAll({where:args});
          }
        },
  
        posts:{
          type : new  graphql.GraphQLList(posts),
          args :{
            id:{
              type: graphql.GraphQLInt
            },
            title:{
              type: graphql.GraphQLString
            },
          },
          resolve(root,args){
            return Db.models.post.findAll({where:args});
          }
        },
  
        tags :{
          type : new  graphql.GraphQLList(tags),
          args :{
            id:{
              type: graphql.GraphQLInt
            },
            name:{
              type: graphql.GraphQLString
            },
          },
          resolve(root,args){
            return Db.models.tag.findAll({where:args});
          }
        }
  
      }
    }
  
  });
  var Schama = new graphql.GraphQLSchema({
    query : query,
    mutation : Mutation
  })
  
  module.exports = Schama;
  </code></pre>