log.txt

You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.each

{"utf8"=>"✓",
 "_method"=>"put",
 "authenticity_token"=>"7dsqpiKslrOcAL+OB1RRhwYpikMn1s9AtBvRWsh2kpA=",
 "work"=>{"name"=>"dsfsdfd",
 "categorization_attributes"=>{"1"=>"1",
 "3"=>"1"},
 "description"=>"dsfsdffdfd"},
 "commit"=>"Update Work",
 "id"=>"5"}

works_controller.rb

def update
    @work = Work.find(params[:id])
    params[:categorization_attributes].each do |k, v| 
      puts "#{k} and #{v}"
    end
    respond_to do |format|
      if @work.update_attributes(params[:work])
        format.html { redirect_to(admin_work_url(@work), :notice => 'Work was successfully updated.') }
        format.xml  { head :ok }
      else
        format.html { render :action => "edit" }
        format.xml  { render :xml => @work.errors, :status => :unprocessable_entity }
      end
    end
  end