application_controller.rb

  def feed_me(source, subtopic)
    @feed = current_user.feeds.build(model_id: source.id, model_name: source.class.to_s, subtopic_id: subtopic.id)
    @feed.save
  end

console

ActiveModel::MassAssignmentSecurity::Error in ScreensController#create

Can't mass-assign protected attributes: subtopic_id
Rails.root: /Users/telagraphic/Documents/programming/rails_projects/codebase

Application Trace | Framework Trace | Full Trace
app/controllers/application_controller.rb:9:in `feed_me'
app/controllers/screens_controller.rb:20:in `create'
Request

Parameters:

{"utf8"=>"✓",
 "authenticity_token"=>"Lirq7qJa2mcgmcnxO54pordlxp3LACg7OgC9bbr1YWQ=",
 "screen"=>{"description"=>"asdfasdf"},
 "commit"=>"New Screen",
 "subtopic_id"=>"14"}

feed.rb

class Feed < ActiveRecord::Base
  attr_accessible :model_id, :model_name, :subtopic_id
  
  belongs_to :user
  belongs_to :subtopic
end

screens_controller.rb

  def create
    @subtopic = Subtopic.find_by_id(params[:subtopic_id])
    @screen = @subtopic.screens.build(params[:screen])
    if @screen.save
      feed_me(@screen, @subtopic)
      redirect_to [topic, @subtopic]
    else
      render 'new'
    end
  end

subtopic.rb

class Subtopic < ActiveRecord::Base
  attr_accessible :title
  has_many :feeds
end