title: save current_user.id into another column
body: <p>currently I have the model commodity_group, there are created_by, and updated_by columns in it. It belongs to user, and one user will have commodity_groups. Now, I want to whenever a user created a commodity group, his id will be saved into the created_by, and whenever somebody update a commodity group, his id will be saved into the field update_by. 
  At the moment, i get this error: </p>
  
  <blockquote>
    <p>unknown attribute 'user_id' for CommodityGroup.</p>
  </blockquote>
  
  <p>basically, I don't want to add the column user_id to the commodity_group table since it is the same with the column created_by. Therefore, could somebody guide me here a little bit. Here are my files:</p>
  
  <p><strong>commodity_groups_controller.rb:</strong></p>
  
  <pre><code>class CommodityGroupsController &lt; ApplicationController
    before_action :set_commodity_group, only: [:show, :edit, :update, :destroy]
  
    # GET /commodity_groups
    def index
      @commodity_groups = CommodityGroup.all
    end
  
    # GET /commodity_groups/1
    def show
    end
  
    # GET /commodity_groups/new
    def new
      @commodity_group = current_user.commodity_groups.build
    end
  
    # GET /commodity_groups/1/edit
    def edit
    end
  
    # POST /commodity_groups
    def create
      @commodity_group = current_user.commodity_groups.build(commodity_group_params)
  
  
      if @commodity_group.save
        redirect_to commodity_groups_path, notice: init_message(:success, t('message.new_success', page_name: t('page_name.commodity_group')))
      else
        redirect_to new_commodity_group_path, notice: init_message(:error, t('message.new_error', page_name: t('page_name.commodity_group')))
      end
    end
  
    # PATCH/PUT /commodity_groups/1
    def update
      if @commodity_group.update(commodity_group_params)
        redirect_to @commodity_group, notice: 'Commodity group was successfully updated.'
      else
        render :edit
      end
    end
  
    # DELETE /commodity_groups/1
    def destroy
      @commodity_group.destroy
      redirect_to commodity_groups_url, notice: 'Commodity group was successfully destroyed.'
    end
  
    private
      # Use callbacks to share common setup or constraints between actions.
      def set_commodity_group
        @commodity_group = CommodityGroup.find(params[:id])
      end
  
      # Only allow a trusted parameter "white list" through.
      def commodity_group_params
        params[:commodity_group]
      end
  end
  </code></pre>
  
  <p><strong>commodity_group.rb</strong>:</p>
  
  <pre><code>class CommodityGroup &lt; ActiveRecord::Base
    extend FriendlyId
    friendly_id :code, use: :history
  
    belongs_to :user_created,
               class_name: 'User',
               primary_key: :id,
               foreign_key: :created_by
    belongs_to :user_updated,
               class_name: 'User',
               primary_key: :id,
               foreign_key: :updated_by
  
    validates_presence_of :code
    validates_presence_of :name
    validates_presence_of :user
  end
  </code></pre>
  
  <p><strong>user.rb:</strong></p>
  
  <pre><code>class User &lt; ActiveRecord::Base
    extend FriendlyId
    include Filterable
  
    friendly_id :slug_candidates, use: :history
    has_secure_password
    acts_as_paranoid
  
    has_many :activities
    has_many :pricing_histories
    has_many :commodity_groups
  end
  </code></pre>