Here's a little trick I found when handling multiple nested models in a form that require validations for each model.

Spec time. For example, we have a model, Author, which has_many Books and has_many Magazines. Book and Magazine has validations. On a single page, we create a three forms for Book, Magazine, and Author. There is also an option select that a user can choose if the Author has a Book or a Magazine.

Pretty straight forward. Let's do some code.

#models/author.rb

class Author < ActiveRecord::Base

  has_many :books

  has_many :magazines


  accepts_nested_attributes_for :books

  accepts_nested_attributes_for :magazines

  validates_presence_of :name

end

#models/book.rb

class Book < ActiveRecord::Base

  belongs_to :author

  validates_presence_of :title, 

                        :genre

end

#models/magazine.rb

class Magazine < ActiveRecord::Base

  belongs_to :author

  validates_presence_of :title, 

                        :genre

end

Basic model setup with validations and accepts_nested_attributes_for. More info on that can be found in the Ruby on Rails API - Nested Attributes.

Moving on to the form setup..

#views/authors/new.html.erb

<% form_for @author do |f| %>

<%= f.error_messages %>

  


    <%= f.label :name %>


    <%= f.text_field :name %>

  

  


    <%= f.radio_button("media_type", "book" %><%= f.label :media_type_book, 'Book' %> [?]

    <%= f.radio_button("media_type", "magazine" %><%= f.label :media_type_magazine, 'Magazine' %> [?]

  

  <% f.fields_for :books do |book| %>

    


      <%= book.label :title %>


      <%= book.text_field :title %>

    

    


      <%= book.label :genre %>


      <%= book.text_field :genre %>

    

  <% end %>


  <% f.fields_for :magazines do |magazine| %>

    


      <%= magazine.label :title %>


      <%= magazine.text_field :title %>

    

    


      <%= magazine.label :genre %>


      <%= magazine.text_field :genre %>

    

  <% end %>

  <%= f.submit 'Submit' %>

<% end %>

  

That sets up the form with the nested model, now for the controller code.

#controllers/authors_controller.rb

class AuthorsController < ApplicationController

  def new

    @author = Author.new

    books = @author.books.build #this builds the nested form in the view

    magazines = @author.magazines.build #this builds the nested form in the view

  end


  def create

    param_hash = params[:author]

    if params[:author][:media_type] == "book"

      param_hash.delete("books_attributes")

    elsif params[:author][:media_type] == "magazine"

      param_hash.delete("magazines_attributes")

    end

    @author = Author.new(param_hash)

    @author.save

  end

end

Instead of passing the params straight to Author.new, it's put into a hash variable. Then the params get checked for a title and genre, if empty delete the key from the hash and pass to Author.new. Activerecord wont see the book param and will skip the validations for it.