Auto wrap parameters in a nested hash to simplify requests

Chris I.

Follow

Sep 29, 2020 · 2 min read

If you’ve ever wondered why sending this in your request.

{"name":"Coffee & Code", "location":"Chelsea"}

Becomes this in your controller params.

{
 "name" => "Coffee & Code", 
 "location" => "Chelsea", 
 "meetup" => {
    "name" => "Coffee & Code", 
    "location" => "Chelsea"
 }
}

It’s because of ParamsWrapper.

Note how “name” and “location” exist twice, once inside “meetup” and once outside.

A Quick Demonstration

Let’s create a simple Rails app and review how params changes with and without ParamsWrapper.

Create a new Rails app on the command line.

$ rails new params_app --api --skip-unit-test --database=postgres

Add a controller called /controllers/meetups_controller.rb, and paste in the following.

class MeetupsController < ApplicationController
  def create
    render json: {}, status: 200
  end
end

Edit /config/routes.rb.

Rails.application.routes.draw do
  post "meetups", to: "meetups#create"
end

And make a request from the command line.

curl -XPOST http://localhost:3000/meetups \
-H 'Content-Type: application/json' \
-d '{"name":"Coffee & Code", "location":"Chelsea"}'

The Rails server logs will now display below.

Started POST "/meetups" for ::1 at 2020-09-29 12:11:40 -0400
Processing by MeetupsController#create as */*
  Parameters: {
    "name"=>"Coffee & Code", 
    "location"=>"Chelsea", 
    "meetup"=>{
        "name"=>"Coffee & Code", 
        "location"=>"Chelsea"
    }
}

Completed 200 OK in 0ms (Views: 0.1ms | ActiveRecord: 0.0ms | Allocations: 139)

We didn’t specify a “meetup” key/value in the request, so how did it get there?

ActiveRecord’s ParamsWrapper did that automatically. It’s default behavior is to wrap requests in a nested hash, based on the name of the resource.

Turn ParamsWrapper Off

The default is set in /initializers/wrap_parameters.rb.

ActiveSupport.on_load(:action_controller) do
  wrap_parameters format: [:json]
end

But it can be turned off by changing [:json] to [] in the initializer, or inside a specific controller.

After deactivating, controller params look like this.

{
  "name"=>"Coffee & Code", 
  "location"=>"Chelsea"
}

The Benefit of ParamsWrapper

It allows you to send requests like this.

{"name":"Coffee & Code", "location":"Chelsea"}

Instead of this.

{"meetup": {"name":"Coffee & Code", "location":"Chelsea"}}

Strong params (see below), requires the root element “meetup”, so this makes it slightly easier on the requester.

params.require(“meetup”).permit(“name”, “location”)

That said, the net benefit is debatable given the ambiguity it adds, but that’s Rails for you :)

I hope you found this useful. Let me know if you have related questions!

Rails 6.0.3.2 was used for the above code.