Question

I am in rails 3.1. I have the following models

    class Tool < ActiveRecord::Base
        has_many :comments
    end

    class Comment < ActiveRecord::Base
        belongs_to :tool
        has_many :relationships
        has_many :advantages, :through => :relationships, :source => :resource, :source_type => 'Advantage'
        has_many :disadvantages, :through => :relationships, :source => :resource, :source_type => 'Disadvantage'

    end

    class Relationship < ActiveRecord::Base
        belongs_to :comment
        belongs_to :resource, :polymorphic => true
    end

    class Disadvantage < ActiveRecord::Base
        has_many :relationships, :as => :resource
        has_many :comments, :through => :relationships
    end

    class Advantage < ActiveRecord::Base
        has_many :relationships, :as => :resource
        has_many :comments, :through => :relationships
    end

In short, A Tool has many comments. A Comment inturn is associated with Advantages and Disadvantages. So in my tool/show page, I would list out all the comments.

But if I have to add comment to the tool page, there would be a form which has a textarea for comment and two multi select list boxes for advantages and disadvantages.

And here is the catch, if the user wanna select from existing adv/disadv, the user can select from the list box or if the user wants to a add a new adv/disadv he can type it and add it, so that it is saved thru an ajax call and the new adv/disadv is added to the list box. How am I supposed to do this?

Answer 1

What you’re looking for are “nested forms” – they are really straight-forward to use.

in your Gemfile add:

gem "nested_form"

1) in your main_model, you’ll include a call to accepts_nested_attributes_for :nested_model

class MainModel
  accepts_nested_attributes_for :nested_model
end

2) in the view for your main_model instead of form_for() , you will call nested_form_for() at the top

= nested_form_for(@main_model) do |f|
   ...

Check the Rails page for that method, it has interesting options, e.g. :reject_if , :allow_destroy, …

3) in the view for the main_model, when you want to show a sub-form for the nested model, you will do

= f.fields_for :nested_model   # replace with your other model name

it will then just use the _form partial for the nested_model and embed it in the view for the main_model

works like a charm!

Check these RailsCast.com episodes, which cover Nested Forms in depth:

http://railscasts.com/episodes/196-nested-model-form-part-1

http://railscasts.com/episodes/197-nested-model-form-part-2

hope this helps