20170116174431_create_release_steps.rb

class CreateReleaseSteps < ActiveRecord::Migration[5.0]
  def change
    create_table :release_steps do |t|
      t.belongs_to :release, index: true
      t.string :name
      t.string :loadbalancer
      t.string :comment
      t.integer :release_id
    end
  end
end

release.rb

# app/model/release.rb
class Release < ApplicationRecord
  has_many :release_steps
end

release_step.rb

# app/model/release_step.rb
class ReleaseStep < ApplicationRecord
  belongs_to :release
end

release_steps_controller.rb

class ReleaseStepsController < ApplicationController
  before_action :set_release
  
  def create
    @release_step = @release.release_steps.create(release_step_params)

    if @release_step.save
      render json: @release_step, status: :created, location: @release_step
    else
      render json: @release_step.errors, status: :unprocessable_entity
    end
  end
  
  private
  
  def set_release
    @release = Release.find(params[:release_id])
  end
end

routes

# config/routes
Rails.application.routes.draw do
  resources :releases do
    resources :release_steps
  end
  # For details on the DSL available within this file, see http://guides.rubyonrails.org/routing.html
end

troubleshooting and debugging output

# HTTP call to create a release step
curl localhost:3000/release_steps -X POST -H "Content-Type: application/json" -d '{"name": "test-name", "loadbalancer":"test-lb","comment":"test-comment"}'


# So...shouldn't I be able to do this in the ReleaseStep controller?
def create
  @release_step = @release.release_steps.create(release_step_params)

  if @release_step.save
    render json: @release_step, status: :created, location: @release_step
  else
    render json: @release_step.errors, status: :unprocessable_entity
  end
end

# Because I get this
NoMethodError: undefined method `release_steps' for nil:NilClass