def create
  @post = Post.new(post_params)

  respond_to do |format|
    if @post.save
      format.html { redirect_to @post, notice: 'Post was successfully created.' }
      format.json { render :show, status: :created, location: @post }
    else
      format.html { render :new }
      format.json { render json: @post.errors, status: :unprocessable_entity }
    end
  end
end

def create
  @post = Post.new(post_params)

  if @post.save
    redirect_to @post, notice: 'Post was successfully updated.' }
  else
    flash[:alert] = "Error updating post: #{@post.errors.full_messages.to_sentence}"
    render :edit
  end
end

def update
  @post = Post.find(params[:id])

  if @post.update(post_params)
    redirect_to @post, notice: 'Post was successfully created.' }
  else
    flash[:alert] = "Error creating post: #{@post.errors.full_messages.to_sentence}"
    render :new
  end
end

def create
  @post = Post.new(post_params)
  crud_respond @post # will call save, set flashes and redirects/render appropriately
end

def update
  @post = Post.find(params[:id])
  @post.assign_attributes(post_params)
  crud_respond @post # also call save
end

def destroy
  @post = Post.find(params[:id])
  crud_respond @post # call destroy because called from destroy action
end

gem 'crud_responder', github: 'olegantonyan/crud_responder'

class ApplicationController
  include CrudResponder
end

def create
  @post = Post.new(post_params)
  crud_respond @post, success_url: root_path, error_action: :custom_new_action
  # will redirect to root_path in case of success or render :custom_new_action otherwise
end

# app/controllers/media_items_controller.rb
class MediaItemsController < ApplicationController
  def create_multiple
    @media_item_multiple = MediaItem::CreateMultiple.new(media_item_create_multiple_params)
    crud_respond @media_item_multiple, success_url: media_items_path
  end

  private

  def media_item_create_multiple_params
    params.require(:media_item_create_multiple).permit(:description, files: [])
  end
end

# app/models/media_item/create_multiple.rb
class MediaItem::CreateMultiple
  include ActiveModel::Model

  attr_accessor :description, :files
  validates :files, presence: true

  def save
    return false unless valid?
    ActiveRecord::Base.transaction do
      files.each do |file|
        MediaItem.create!(description: description,  file: file)
      end
    end
    true
  rescue ActiveRecord::RecordInvalid => e
    errors.add(:base, e.to_s)
    false
  end
end