In Rails, how do I create nested objects using accepts_nested_attributes_for?

Question

In my Rails application, I am attempting to set up creation form for a TrainingClass model. I want this form to allow the user to create multiple ClassMeeting models (which have a belongs_to relationship with the TrainingClass model) within the same form, and I am using accepts_nested_attributes_for to do this. Unfortunately, whenever I submit the form I get the error message: Meetings class can't be blank.

I realize that this is because ClassMeeting has validates :class_id, presence: true, and because TrainingClass can't have an id until after it is saved, but I'm not sure of the right way to get around this. (I can think of a few possible ways, but they aren't exactly elegant solutions.) Any ideas? I'd appreciate any help you can give me.

Note: I realize quite a few questions similar to this have been asked in the past. However, most of those questions are old and have outdated answers, and none of them solved my problem.

---

Here is my code. Keep in mind that although I have simplified some aspects of it for brevity, the relationship between the ClassMeeting and TrainingClass models was left untouched:

ClassMeeting Model:

# == Schema Information
#
# Table name: class_meetings
#
#  id         :integer         not null, primary key
#  class_id   :integer
#  start      :datetime
#  end        :datetime
#  location   :string(255)
#  created_at :datetime        not null
#  updated_at :datetime        not null
#

class ClassMeeting < ActiveRecord::Base
  attr_accessible :start, :end, :location

  validates :class_id, presence: true
  validates :start, presence: true
  validates :end, presence: true
  validates :location, presence: true, length: {maximum: 255}

  belongs_to :training_class, foreign_key: :class_id, inverse_of: :meetings
end

TrainingClass Model:

# == Schema Information
#
# Table name: training_classes
#
#  id            :integer         not null, primary key
#  description   :string(255)
#  created_at    :datetime        not null
#  updated_at    :datetime        not null
#

class TrainingClass < ActiveRecord::Base
  attr_accessible :description, :meetings_attributes

  validates :description, length: {maximum: 255}

  has_many :meetings, class_name: :ClassMeeting, foreign_key: :class_id, inverse_of: :training_class

  accepts_nested_attributes_for :meetings, allow_destroy: true
end

TrainingClasses Controller:

class TrainingClassesController < ApplicationController
  def new
    @training_class = TrainingClass.new()
    @training_class.meetings.build
  end

  def create
    @training_class = TrainingClass.new()

    if @training_class.update_attributes(params[:training_class])
        redirect_to @training_class, notice: 'Class was successfully created.'
    else
        render "new"
    end
  end
end

TrainingClass Form (View):

<%= form_for @training_class do |f| %>
    <%= render 'shared/error_messages', object: f.object %>

    <%= f.text_area :description %>

    <h2>Schedule</h2>
    <%= f.fields_for :meetings do |meeting| %>
        <%= meeting.label :start, "Start of Meeting:" %>
        <%= meeting.text_field :start %>

        <%= meeting.label :end, "End of Meeting:" %>
        <%= meeting.text_field :end %>

        <%= meeting.label :location, "Location:" %>
        <%= meeting.text_field :location %>
    <% end %>

    <%= f.submit class:"btn btn-large btn-primary" %>
<% end %>

Answer 1

All right, I found the solution to my problem. All I need to do is validate the presence of training_class instead of class_id in the ClassMeeting model. That way the existence of a training class is still validated, but the validator doesn't interfere with accepts_nested_attributes_for's method of saving the model:

class ClassMeeting < ActiveRecord::Base
  attr_accessible :start, :end, :location

  validates :training_class, presence: true # :training_class instead of :class_id
  validates :start, presence: true
  validates :end, presence: true
  validates :location, presence: true, length: {maximum: 255}

  belongs_to :training_class, foreign_key: :class_id, inverse_of: :meetings
end

Answer 2

I pored over this example, trying out the differences with my code, but in the end my problem was fixed by using :inverse_of.

See accepts_nested_attributes_for child association validation failing