form_for models with specific relations - ruby-on-rails

Question

I have
Models
class Group < ApplicationRecord
has_many :group_artists
has_many :singers, -> { where role: "Singer" }, class_name: "GroupArtist"
has_many :guitarists, -> { where role: "Guitarist" }, class_name: "GroupArtist"
end
class GroupArtist < ApplicationRecord
belongs_to :group
belongs_to :artist
end
class Artist < ApplicationRecord
has_many :group_artists
has_many :groups, through: :group_artists
end
group_artists table has these columns
class CreateGroupArtists < ActiveRecord::Migration[5.1]
def change
create_table :group_artists, id: false do |t|
t.references :group, foreign_key: true, null: false
t.references :artist, foreign_key: true, null: false
t.string :role
t.string :acting
t.timestamps
end
end
end
Controller
class GroupsController < ApplicationController
def new
#group = Group.new
#singers = #group.singers.build
#guitarists = #group.guitarists.build
#artists = Artist.all // for a selection
end
def create
#group = Group.new(allowed_params)
#group.save
end
private
def allowed_params
params.require(:group).permit(:name, :singers, :guitarists, group_artists_attributes: [:group_id, :artist_id, :role, :acting])
end
end
views/groups/_form.html.erb
<%= form_for #group do |f| %>
<%= f.label "Singers" %>
<%= f.fields_for :singers do |singer| %>
<%= singer.select(:artist_id, #artists.collect { |a| [a.name, a.id.to_i] }, { include_blank: true }) %>
<% end %>
<%= f.label "Guitarists" %>
<%= f.fields_for :guitarists do |guitarist| %>
<%= guitarist.select(:artist_id, #artists.collect { |a| [a.name, a.id.to_i] }, { include_blank: true }) %>
<% end %>
<%= f.submit "Submit" %>
<% end %>
It creates the group all right, but doesn't create the relation in GroupArtist.
I know something's missing in the controller part. I should add something after the ".build" like (role: "Singer") but it doesn't do anything as well.
Ruby -v 2.4.1
Rails -v 5.1.3

Answer 1

Since you are using group_artists as more than just a simple join table you need to use nested attributes to create a row with metadata:
class Group < ApplicationRecord
has_many :group_artists
has_many :singers, -> { where role: "Singer" }, class_name: "GroupArtist"
has_many :guitarists, -> { where role: "Guitarist" }, class_name: "GroupArtist"
accepts_nested_attributes_for :group_artists,
reject_if: ->{|a| a[:artist_id].blank? || a[:role].blank?}
end
Also the structure using different associations to create nested records based on the role of the band members is not really scalable - for every possible role the class / form will swell.
Instead you may want to use two selects:
<%= form_for #group do |f| %>
<fields_for :group_artists do |ga| %>
<div class="field">
<%= f.label :artist_id, "Artist" %>
<%= f.collection_select :artist_id, Artist.all, :id, :name %>
<%= f.label :role %>
<%= f.select :role, %w[ Singer Guitarist ] %>
</div>
<% end %>
<%= f.submit "Submit" %>
<% end %>
Also you are not saving the record in your #create method.
def create
#group = Group.new(allowed_params)
if #group.save
# ...
else
render :new
end
end

Answer 2

You should add this in your controller action create
def create
#group = Group.new(allowed_params)
#group.save
end
Group.new doesn't persist model in DB but save after yes.

Answer 3

These were the adds and changes I needed to make it work.
class Group < ApplicationRecord
has_many :group_artists
has_many :singers, -> { where role: "Singer" }, class_name: "GroupArtist"
has_many :guitarists, -> { where role: "Guitarist" }, class_name: "GroupArtist"
accepts_nested_attributes_for :group_artists,
reject_if: proc { |a| a[:artist_id].blank? || a[:role].blank? },
allow_destroy: true
end
class GroupsController < ApplicationController
def new
#group = Group.new
#group.group_artists.build
#artists = Artist.all // for a selection
end
def create
#group = Group.new(allowed_params)
#group.save
end
end
groups/_form.html.erb
<%= form_for #group do |f| %>
<div class="singers">
<%= f.label :singers %>
<%= f.fields_for :group_artists do |ga| %>
<%= ga.collection_select :artist_id, Artist.all, :id, :name %>
<%= ga.hidden_field :role, value: 'Singer' %>
<% end %>
</div>
<div class="guitarists">
<%= f.label :guitarists %>
<%= f.fields_for :group_artists do |ga| %>
<%= ga.collection_select :artist_id, Artist.all, :id, :name %>
<%= ga.hidden_field :role, value: 'Guitarist' %>
<% end %>
</div>
<%= f.submit "Submit" %>
<% end %>
That way you don't have to specify the artist's role if his name's selected in the singers or guitarists labeled div.
That's it. Thanks to max, for guiding me to the right direction.