Nested forms with ActiveModel::Model objects

ActiveModel::Model is an excellent way to make objects behave like ActiveRecord. One potential use for this is if we want to make use of form data that doesn't necessarily persist to an object, but we want to keep our controllers clean. A great example can be found on this thoughtbot blog post from about a year ago.

One thing that I think many have found lacking, however, is has_many and accepts_nested_attributes_for functionality. I found myself in this boat recently when I wanted to use these form objects to create multiple records. How can I have a form that can create arbitrarily many records and have validation errors appear for each record I'm trying to create, without associations?

Like many, I began looking for gems to solve my problem, but after a day or so of searching for the perfect solution, I found the missing key. If an object with a one-to-many association is instantiated with a params hash, and that hash has a key for the association, Rails will call the <association_name>_attributes= method on that object. So, we need to define that on our form object.

class ContactListForm
  include ActiveModel::Model

  attr_accessor :contacts

  def contacts_attributes=(attributes)
    @contacts ||= []
    attributes.each do |i, contact_params|
      @contacts.push(Contact.new(contact_params))
    end
  end
end

Now we need a form for this model. Keeping it simple:

<div>
  <%= form_for @contact_list, url: contacts_path, method: :post do |f| %>
    <%= f.fields_for :contacts do |c| %>
      <%= c.text_field :first_name %>
      <%= c.text_field :last_name
      <%= c.text_field :phone %>
    <% end %>

    <p>
      <%= f.submit "Submit" %>
    </p>
  <% end %>
</div>

Of course our object doesn't have any contacts yet, so our controller will need to make sure that the form has at least one fields_for block to render by giving it one on initialization

class ContactsController < ApplicationController
 def new
    @contact_list = ContactListForm.new(contacts: [Contact.new])
  end
end

My controller also needs a create action for when the form is submitted.

class ContactsController < ApplicationController
  def create
    @contact_list = ContactListForm.new(params[:contact_list_form])

    if @contact_list.save
      flash[:notice] = "Created contacts"
      redirect_to root_path
    else
      flash[:notice] = "There were errors"
      render :action => 'new'
    end
  end
end

If all forms were valid, then all contacts were submitted to the database. But what about validation errors for each contact on the form? First I took some good advice from a StackOverflow post to just redefine the error_messages method that was deprecated back in Rails 3

class CustomFormBuilder < ActionView::Helpers::FormBuilder
    def error_messages
      return unless object.respond_to?(:errors) && object.errors.any?

      html = ""
      html << @template.content_tag(:span, "Please correct the following errors.")
      html << object.errors.full_messages.map { |message| @template.content_tag(:li, message) }.join("\n")

      @template.content_tag(:ul, html.html_safe)
    end
end

ActionView::Base.default_form_builder = CustomFormBuilder

Now just a small update to my form:

<%= f.fields_for :contacts do |c| %>
  <%= c.error_messages %>
  <%= c.text_field :first_name %>
  <%= c.text_field :last_name
  <%= c.text_field :phone %>
<% end %>

And I'm good to go.

For my next post, we'll add a link to the form that will add inputs for another contact, so that we may submit an arbitrary number of contacts. Please stay tuned!