Question

How can I generate form fields for a has_many :through association that has additional attributes?

The has_many :through relationship has an additional column called weight.

Here’s the migration file for the join table:

create_table :users_widgets do |t|
  t.integer :user_id
  t.integer :widget_id
  t.integer :weight

  t.timestamps
end

The models look like this:

User
  has_many :widgets, :through => :users_widgets,
           :class_name => 'Widget',
           :source => :widget
  has_many :users_widgets
  accepts_nested_attributes_for :widgets # not sure if this is necessary


Widget
  has_many :users, :through => :users_widgets,
           :class_name => 'User',
           :source => :user
  has_many :users_widgets
  accepts_nested_attributes_for :users # not sure if this is necessary


UsersWidget
  belongs_to :user
  belongs_to :widget

For the sake of simplicity, Widget and User only have one field of their own called name, ergo User.first.name and Widget.first.name

Questions:

• How would I append a dropdown selection for Widgets with the corresponding weight to the User create/edit form?

• How can I dynamically add additional Widget forms to Users or User forms to Widgets to easily add or remove these relationships? The nested_form_for gem seems to be the perfect solution but I haven’t been able to get it working.

• Apart from the models and the form partials, are there any changes that need to be made to my controller?

---

Quick note.. I’m not interested in creating new Widgets in the User form or new Users in the Widget form, I only want the ability to select from existing objects.

I’m running Rails 3.1 and simple_form 2.0.0dev for generating my forms.

Answer 1

I will be solving your problem using cocoon, a gem I created to handle dynamically nested forms. I also have an example project to show examples of different types of relationships.

Yours is not literally included, but is not that hard to derive from it. In your model you should write:

class User 
  has_many :users_widgets
  has_many :widgets, :through -> :user_widgets

  accepts_nested_attributes_for :user_widgets, :reject_if => :all_blank, :allow_destroy => true

  #...
end

Then you need to create a partial view which will list your linked UserWidgets. Place this partial in a file called users/_user_widget_fields.html.haml:

.nested-fields
  = f.association :widget, :collection => Widget.all, :prompt => 'Choose an existing widget'
  = f.input :weight, :hint => 'The weight will determine the order of the widgets'
  = link_to_remove_association "remove tag", f

In your users/edit.html.haml you can then write:

= simple_form_for @user do |f|
  = f.input :name

  = f.simple_fields_for :user_widgets do |user_widget|
    = render 'user_widget_fields', :f => user_widget
  .links
    = link_to_add_association 'add widget', f, :user_widgets

Hope this helps.