HowTo: Increment the value of an integer based field in GraphQL

So, I wish to create a GraphQL mutation, in GraphCool, that increments the existing value of an integer field by a predetermined amount, passed in as a variable. For example:

mutation updateLikes ($id: ID!, $count: Int) {
  updatePosts (id: $id, likes: incrementBy $count) {
    id
    likes
  }
}

Query variable

{
  "id" : "cj0qkl04vep8k0177tky596og",
  "count": 1
}

How do I do this?

Also, I am attempting to trigger the above mentioned process from a onClick event, but am getting an 'cannot read property 'props' of null' error message, I suspect on this.props.client.query:

Photo.js

import React from 'react';
import Comments from './Comments';
import CSSTransitionGroup from 'react-addons-css-transition-group';
import { Link } from 'react-router';
import {
  gql,
  graphql,
  withApollo
} from 'react-apollo';
import ApolloClient from 'apollo-client';

class Photo extends React.Component {

  incrementQuery(currentID) {
    console.log("like of id=" + currentID+ " has been incremented by 1");

    this.props.client.query({
      query: gql`
        mutation updatePosts($id: ID!) {
          updatePosts (id: $id, likes: 1) {
            id
            likes
          }
        }  
      `,
      variables: {
        id: currentID,
      },
    });
  }

  render() {
    const { post, i } = this.props;

    return (
      <figure key={i} className="grid-figure">

        <div className='grid-photo-wrap'>
          <Link to={`/view/${post.id}`}>
            <img className='grid-photo' src={post.displaysrc} alt={post.caption} />
          </Link>

          <CSSTransitionGroup transitionName="like" transitionEnterTimeout={500} transitionLeaveTimeout={500}>
            <span key={post.likes} className="likes-heart">{post.likes}</span>
          </CSSTransitionGroup>

        </div>

        <figcaption>
          <p>{post.caption}</p>

          <div className="control-buttons">
            <button onClick={this.incrementQuery.bind(null,i)} className="likes">&hearts; {post.likes}</button>

            <Link to={`/view/${post.id}`} className="button">
              <span className="comment-count">
                <span className="speech-bubble"></span> {(post.comments ? Object.keys(post.comments).length : 0)}
              </span>
            </Link>
          </div>

        </figcaption>

      </figure>
    )
  }
};

Photo.PropTypes = {
  client: React.PropTypes.instanceOf(ApolloClient).isRequired,
  query: React.PropTypes.object,
  variables: React.PropTypes.object,
  data: React.PropTypes.shape({
    loading: React.PropTypes.bool.isRequired,
    error: React.PropTypes.bool.isRequired,
    updatePosts: React.PropTypes.object,
  }).isRequired,
}

const PhotoWithApollo = withApollo(Photo);

export default PhotoWithApollo;

The thrown error is as follows:

Uncaught TypeError: Cannot read property 'props' of null
    at incrementQuery (http://localhost:7770/static/bundle.js:56347:12)
    at proxiedMethod (http://localhost:7770/static/bundle.js:54898:31)
    at HTMLUnknownElement.wrapped (http://localhost:7770/static/bundle.js:47347:30)
    at Object.ReactErrorUtils.invokeGuardedCallback (http://localhost:7770/static/bundle.js:6346:17)
    at executeDispatch (http://localhost:7770/static/bundle.js:6146:22)
    at Object.executeDispatchesInOrder (http://localhost:7770/static/bundle.js:6169:6)
    at executeDispatchesAndRelease (http://localhost:7770/static/bundle.js:5599:23)
    at executeDispatchesAndReleaseTopLevel (http://localhost:7770/static/bundle.js:5610:11)
    at Array.forEach (native)
    at forEachAccumulated (http://localhost:7770/static/bundle.js:6446:10)
    at Object.processEventQueue (http://localhost:7770/static/bundle.js:5815:8)
    at runEventQueueInBatch (http://localhost:7770/static/bundle.js:6475:19)
    at Object.handleTopLevel [as _handleTopLevel] (http://localhost:7770/static/bundle.js:6491:6)
    at handleTopLevelWithoutPath (http://localhost:7770/static/bundle.js:16697:25)
    at handleTopLevelImpl (http://localhost:7770/static/bundle.js:16677:4)
    at ReactDefaultBatchingStrategyTransaction.perform (http://localhost:7770/static/bundle.js:8643:21)
    at Object.batchedUpdates (http://localhost:7770/static/bundle.js:12680:20)
    at Object.batchedUpdates (http://localhost:7770/static/bundle.js:8148:21)
    at dispatchEvent (http://localhost:7770/static/bundle.js:16808:21)
    at HTMLDocument.wrapped (http://localhost:7770/static/bundle.js:47347:30)

Solution

Currently you need to first query the value:

query Post($id: ID!) {
  Post (id: $id) {
    id
    likes
  }
}

Then increment the likes client-side and update the post with the new value for likes:

mutation updateLikes ($id: ID!, $likes: Int!) {
  updatePost (id: $id, likes: $likes) {
    id
    likes
  }
}

Incrementing the likes by a special number is a great use case for a custom mutation, which we are already working on.