title: Graphql mutation returning null, but database is updated
body: <p>I'm trying to add a mutation to allow for the client to add a document to the LineItem schema. The code I've written below allows me to do that when I test it using GraphiQL, but the response I get is null. How do I fix the code so that the response is the new document?</p>
  
  <pre><code>addLineItem: {
      type: LineItemType,
      args: {
        orderId: {type: new GraphQLNonNull(GraphQLID)},
        productId: {type: new GraphQLNonNull(GraphQLID)},
        quantity: {type: new GraphQLNonNull(GraphQLInt)}
      },
      resolve(parent, args) {
        Product.findById(args.productId, (err, result) =&gt; {
          let price = result.price;
          let subtotal = price*args.quantity;
          let lineitem = new LineItem({
            orderId : args.orderId,
            productId : args.productId,
            quantity : args.quantity,
            subtotal: subtotal
          });
          return lineitem.save();
      }}
    }
  },
  </code></pre>