PullMonkey Blog


05 Aug

Dynamic Select Boxes (many to many) - Ruby on Rails


I just got comment asking how one would go about doing a many to many relation in this dynamic select box example. For example, what if an artist belongs to multiple genres. Here we go:

The original tutorial.

Create your models and build your migrations:

1
2
3
4
5

ruby script/generate model genre name:string
ruby script/generate model artist name:string   # no genre_id here, moved to association table
ruby script/generate model song title:string artist_id:integer
ruby script/generate model artist_association artist_id:integer genre_id:integer

Populate your genres, artists and songs through a migration:

1
2

ruby script/generate migration create_hierarchy

Contents of migration:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46

class CreateHierarchy < ActiveRecord::Migration
  def self.up
    # same genres as before
    g1 = Genre.create(:name => "Genre 1")
    g2 = Genre.create(:name => "Genre 2")
    g3 = Genre.create(:name => "Genre 3")

    # same artists as before, but without a genre_id
    a1 = Artist.create(:name => "Artist 1")
    a2 = Artist.create(:name => "Artist 2")
    a3 = Artist.create(:name => "Artist 3")
    a4 = Artist.create(:name => "Artist 4")
    a5 = Artist.create(:name => "Artist 5")
    a6 = Artist.create(:name => "Artist 6")

    # now set which artists belong to which genres
    # Artist 1 belongs to all three genres
    ArtistAssociation.create(:genre_id => g1.id, :artist_id => a1.id)
    ArtistAssociation.create(:genre_id => g2.id, :artist_id => a1.id)
    ArtistAssociation.create(:genre_id => g3.id, :artist_id => a1.id)
   # the rest of the artists only belong to one association
    ArtistAssociation.create(:genre_id => g1.id, :artist_id => a2.id)
    ArtistAssociation.create(:genre_id => g2.id, :artist_id => a3.id)
    ArtistAssociation.create(:genre_id => g2.id, :artist_id => a4.id)
    ArtistAssociation.create(:genre_id => g3.id, :artist_id => a5.id)
    ArtistAssociation.create(:genre_id => g3.id, :artist_id => a6.id)

    Song.create(:title => "Song 1",  :artist_id => a1.id)
    Song.create(:title => "Song 2",  :artist_id => a1.id)
    Song.create(:title => "Song 3",  :artist_id => a2.id)
    Song.create(:title => "Song 4",  :artist_id => a2.id)
    Song.create(:title => "Song 5",  :artist_id => a3.id)
    Song.create(:title => "Song 6",  :artist_id => a3.id)
    Song.create(:title => "Song 7",  :artist_id => a4.id)
    Song.create(:title => "Song 8",  :artist_id => a4.id)
    Song.create(:title => "Song 9",  :artist_id => a5.id)
    Song.create(:title => "Song 10", :artist_id => a5.id)
    Song.create(:title => "Song 11", :artist_id => a6.id)
    Song.create(:title => "Song 12", :artist_id => a6.id)
  end

  def self.down
# you can fill this in if you want.
  end
end

So now we need to populate the database:

1
2

rake db:migrate

Now we need to modify our models to set up the associations.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21

class Genre < ActiveRecord::Base
  has_many :artist_associations
  has_many :artists, :through => :artist_associations
  # CAN"T NEST HMTs .....  has_many :songs, :through => :artists
  # do it by hand ... argh
  def songs
    artists.map{|a| a.songs}.flatten
  end
end

class Artist < ActiveRecord::Base
  has_many :artist_associations
  has_many :genres, :through => :artist_associations
  has_many :songs
end

class ArtistAssociation < ActiveRecord::Base
  belongs_to :artist
  belongs_to :genre
end

That should be it for the many to many relationship.



Everything else is the same as in the last tutorial.

# the controller

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30

class TestItController < ApplicationController
  def index
    @genres  = Genre.find(:all)
    @artists = Artist.find(:all)
    @songs   = Song.find(:all)
  end

  def update_artists
    # updates artists and songs based on genre selected
    genre = Genre.find(params[:genre_id])
    artists = genre.artists
    songs   = genre.songs

    render :update do |page|
      page.replace_html 'artists', :partial => 'artists', :object => artists
      page.replace_html 'songs',   :partial => 'songs',   :object => songs
    end
  end

  def update_songs
    # updates songs based on artist selected
    artist = Artist.find(params[:artist_id])
    songs  = artist.songs

    render :update do |page|
      page.replace_html 'songs', :partial => 'songs', :object => songs
    end
  end
end

Now as far as views go we have one view (index.html.erb) and two partials (_songs and _artists). Let's take a look at those:

# the _songs partial (_songs.html.erb):

1
2
3

<%= collection_select(nil, :song_id, songs, :id, :title,
                     {:prompt   => "Select a Song"}) %>

# the _artists partial (_artists.html.erb):

1
2
3
4
5
6

<%= collection_select(nil, :artist_id, artists, :id, :name,
                     {:prompt   => "Select an Artist"},
                     {:onchange => "#{remote_function(:url  => {:action => "update_songs"},
                                                      :with => "'artist_id='+value")}"}) %>
<br/>

# and last, but not least, the index view (index.html.erb):

1
2
3
4
5
6
7
8
9

<%= javascript_include_tag :defaults %>
<%= collection_select(nil, :genre_id,  @genres,  :id, :name,
                      {:prompt   => "Select a Genre"},
                      {:onchange => "#{remote_function(:url  => {:action => "update_artists"},
                                                       :with => "'genre_id='+value")}"}) %>
<br/>
<div id="artists"><%= render :partial => 'artists', :object => @artists %></div>
<div id="songs"><%= render :partial => 'songs',   :object => @songs %></div>