更新has_many：通过与Rails表单关联

Question

I'm trying to update the form for a Song model that has multiple Genres through a song_genre_relationships model. I'd prefer to update the genre id's for any given song through an array in the form field.

I suspect my controller is off, but I'm still pretty new to Rails and am unsure.

Here are my models, form and controller.

EDITED BASED ON meagar's RESPONSE BELOW.

song.rb

class Song < ActiveRecord::Base

    has_many :song_genre_relationships
    has_many :genres, :through => :song_genre_relationships

end

song_genre_relationships.rb

class SongGenreRelationship < ActiveRecord::Base

    belongs_to :song
    belongs_to :genre

end

genre.rb

class Genre < ActiveRecord::Base

    has_many :song_genre_relationships
    has_many :songs, :through => :song_genre_relationships

end

song_controller.rb

class SongController < ApplicationController

    def update
    @song = Song.find(params[:id])

    relationships = @song.song_genre_relationships
    @genres = Genre.all
    @genres.each do |genre|
        unless relationships.detect { |g| g.genre_id == genre.id }
            relationships.build genre_id: genre.id
        end
    end

    if @song.update_attributes(song_params)

        flash[:notice] = "Song updated successfully."

        redirect_to(:action => 'index')
    else
        render('edit')
    end
end

private
def song_params
    params.require(:song).permit(:name, :genre_ids => [])
end

_form.html.erb (song form)

<%= form_for(@song) do |f| %>

    <%= f.label :name, 'Title' %>
    <%= f.text_field :name %>

    <%= f.label :genre_ids, 'Genres' %>
    <%= f.text_area :genre_ids %>


<%= f.submit %>
<% end %>

Answer 1

This is wrong:

  @genres = params[:song][:genre_ids]
    Array(@genres).each do |genre|
        unless relationships.detect { |g| g.genre_id == genre.id }
            relationships.build genre_id: genre
        end
    end

params[:song][:genre_ids] is a string. It contains, in your example, "[<whatever id's input to the form>]" .

You're wrapping it in an array with Array(@genres) . Now, you have an array containing a single item, which is the string. Then you iterate over that single item, placing it into genre inside the loop

Then you test g.genre_id == genre.id . genre is that string, and you're invoking .id on it, hence your error.

It's impossible to tell what you're trying to do, but you definitely cannot invoke .id on a string and expect to get the ID of some database record back.