I haven''t been able to get this to work for some reason, and its driving me nuts, and help is greatly appreciated. So here''s the deal: I''m have a table called Songs Each song has_many versions called Production_edits each production_edit has its own instrumentation. Instruments has_and_belongs_to_many Production_edits and vice versa. and i have my instruments_production_edits join table In my new song form, you can add multiple production_edits, the only problem is that since I''m using HABTM checkboxes for the instrumentation, for every instrument i check, rails interprets it as a seperate production_edit. Here''s my view in for the new song form: ... <% form_for(@song, :url => {:action => ''create_song''}, :html => { :multipart => true } ) do |f|%> <%= error_messages_for :song %>... (below is in my _production_edit partial) <% @production_edit = production_edit %> <% fields_for "song[production_edit_attributes][]", production_edit do |production_edit_form| %> <%= error_messages_for :production_edit %> <%= production_edit_form.text_field :title %> (etc...) <% for instrument in Instrument.find(:all) %> <%= check_box_tag "song[production_edit_attributes][][instrument_ids] []", instrument.id, production_edit.instruments.include?(instrument) %> <%= instrument.name %><br/> <% end %> <% end %> Then in my song.rb model i have: ... validates_associated :production_edits def production_edit_attributes=(production_edit_attributes) production_edit_attributes.each do |attributes| production_edits.build(attributes) end end ... When i fill out the form with each production_edit having more than one instrument i get something like this in my parameters: "production_edit_attributes"=>[{"title"=>"", "instrument_ids"=>[]}, {"title"=>"", "instrument_ids"=>[]}, {"instrument_ids"=>["1"]}, {"instrument_ids"=>["3"]}, {"instrument_ids"=>["1"]}, {"instrument_ids"=>["2"]}] when I''m actually trying to achieve this: "production_edit_attributes"=>[{"title"=>"", "instrument_ids"=> ["1,"3"]}, {"title"=>"", "instrument_ids"=>["1","2"]}] I''ve also tried something similar with a multiple select box, however I get the same result in the params. Any ideas? or let me know if I''m not clear enough on what I''m trying to do.