Uploading files using Relay and a Rails GraphQL server

I recently needed to upload a file from a React-Relay App to my GraphQL Rails server and found out it was not as straight forward as I thought and not very documented. Thought I’d share what I have done to make file uploading work with Rails and Relay. Big thanks to Robert Mosolgo who guided me towards this solution.

I you haven’t checked out my first blog about GraphQL you can see it here.


Relay Mutation have a method called getFiles which allows you to send files to a server. It should return a map of files like this {[key: string]: File} Here’s a quick example of how we could use it:

// A react component to upload a file
class FileUploader extends React.Component {
  onSubmit() {
    const name = this.refs.name.value;
    const file = this.refs.fileInput.files.item(0);
      new UploadFileMutation({
        name: name,
        file: file,

// The actual mutation
class UploadFileMutation extends Relay.Mutation {
  getFiles() {
    return {
      file: this.props.file,

  // ... Rest of your mutation

GraphQL Rails API

The part that gets more confusing is how to receive that file server side. If you’re using GraphQL-js there is this example that explains how you would design your schema to receive files.

Unfortunatly this wasn’t really possible using GraphQL-ruby and I had to find another way to do it. Luckily, GraphQL-ruby passes a context argument to the nodes when it resolves a query. That means we can pass the files along to the query in the controller and retrieve it at resolve time in the mutation.

Here’s how you could write your controller which in my case is called QueriesController.

module V1
  class QueriesController < ApplicationController

    def create
      query_string = params[:query]
      query_variables = ensure_hash(params[:variables]) || {}

      query = GraphQL::Query.new(
        variables: query_variables,
        context: { file: request.params[:file] }

      render json: query.result

    # If the request wasn't `Content-Type: application/json`, parse the variables:
    def ensure_hash(variables_param)
      variables_param.kind_of?(Hash) ? variables_param : JSON.parse(variables_param)

You’ll notice that there is an ensure_hash method added here. Variables in Relay mutations with files aren’t sent with a Content-Type: application/json so they arrive as a String in your controller. Here we simply check if the variables are sent as a Hash and if they’re not we parse them as JSON.

The rest is pretty straight forward, we add the file param to our context so it can be passed along during resolve. That means that our Mutation on the server side can look a little bit like this:

AddFileMutation = GraphQL::Relay::Mutation.define do
  name "AddFile"
  input_field :name, !types.String

  # ... Add your return_field here

  # Here's the mutation operation:
  resolve -> (args, ctx) {
    file = ctx[:file]
    raise StandardError.new("Expected a file") unless file
    # ... Do what you want with the file!

Here you have it! Note that you should probably not be uploading large files on the server and you might want to upload it browser side!

Hope it’ll help some of you!

You can find me on Twitter @__xuorig__ or Github

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