Validations For Multiple Nested Model Forms

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 hasmany Books and hasmany 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 acceptsnestedattributes_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 %>
  <p>
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  </p>
  <p>
    <%= f.radio_button("media_type", "book" %><%= f.label :media_type_book, 'Book' %> [?]
    <%= f.radio_button("media_type", "magazine" %><%= f.label :media_type_magazine, 'Magazine' %> [?]
  </p>
  <% f.fields_for :books do |book| %>
    <p>
      <%= book.label :title %><br />
      <%= book.text_field :title %>
    </p>
    <p>
      <%= book.label :genre %><br />
      <%= book.text_field :genre %>
    </p>
  <% end %>

  <% f.fields_for :magazines do |magazine| %>
    <p>
      <%= magazine.label :title %><br />
      <%= magazine.text_field :title %>
    </p>
    <p>
      <%= magazine.label :genre %><br />
      <%= magazine.text_field :genre %>
    </p>
  <% 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.

Published 06 Mar 2010

Software developer working with Ruby, Go, and JavaScript. Hobbyist street photographer and climber.

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