haml - Rails: submit form with nested attributes throws 'couldn't find entity with id=xxx' -

i got following model classes:

class movie < activerecord::base   has_and_belongs_to_many :actors   accepts_nested_attributes_for :actors, :allow_destroy => true   ... end 

and

class actor < activerecord::base   has_and_belongs_to_many :movies   ... end 

movies/_form.html.haml view contains nested form actors:

... = form_for @movie |movie_form|   = movie_form.fields_for :actors |actor_form|     = actor_form.text_field :id, "id"     = link_to_remove_fields "remove", actor_form   = link_to_add_fields movie_form :actors ...   = movie_form.submit 'save' 

link_to_remove_fields , link_to_add_fields helper methods performing javascript calls, adding new fields new actors or removing actors.

the controller:

   class moviecontroller < applicationcontroller       #post form      def create        @movie = movie.new(params[:movie])        ...      end       #get empty form 1 field actor      def new        @movie = movie.new        1.times {@movie.actors.build}         respond_to |format|          format.html          format.json { render json: @movie }        end      end    end 

the problem: when 'save' pressed (with 1 actor id=718, example) , form submitted, @movie = movie.new(params[:movie]) line of controller throws following error:

couldn't find actor id=718 movie id=

i'm sure there entity id=718 in actor database.

the easiest thing if trying associating existing actors new movie not use accepts_nested_attributes @ all.

one of things actors associations actor_ids, return ids of associated actors, or allows set list of associated actors.

to submit array via rails form, name of input must end [], example if have input name movie[actor_ids][] , value 1, you'll get

{'movie' => {'actor_ids' => ['1']}} 

in controller. if want submit second actor, create input same name movie[actor_ids][]. use select box multiple attribute set, can bit unfriendly users when there more few options pick from.