CreatePostMutation.js

// 1
import {
  commitMutation,
  graphql,
} from 'react-relay'
import {ConnectionHandler} from 'relay-runtime'
import environment from './Environment'
// 2
const mutation = graphql`
  mutation CreatePostMutation($input: CreatePostInput!) {
    createPost(input: $input) {
      post {
        id
        description
        imageUrl
      }
    }
  }
`
// 3
export default (description, imageUrl, viewerId, callback) => {
  // 4
  const variables = {
    input: {
      description,
      imageUrl,
      clientMutationId: ""
    },
  }
// 5
  commitMutation(
    environment,
    {
      mutation,
      variables,
      // 6
      optimisticUpdater: (proxyStore) => {
        // ... you'll implement this in a bit
      },
      updater: (proxyStore) => {
        // ... this as well
      },
      // 7
      onCompleted: () => {
        callback()
      },
      onError: err => console.error(err),
    },
  )
}