How to build and save Parent and Child records when required = true?

Question

Given the following models

class Parent
  has_many :children
end
class Child
  belongs_to :parent, required: true
end

Is it possible to create Parent and Children at the same time?

@parent = Parent.new(valid_attributes)
@parent.children.build(valid_attributes)
@parent.save
@parent.errors.messages 
#=> {:"children.parent"=>["must exist"]}

Removing the required: true allows the record to save. But is there a way to enable parents and children to be saved together while still validating parent exists?

Answer 1

You can use accepts_nested_attributes_for, Enabling nested attributes on association allows you to create the parent and child in one go.

Model parent.rb

class Parent < ActiveRecord::Base
  has_many :children

  #enable nested attributes
  accepts_nested_attributes_for :children
end

Model child.rb

class Child < ActiveRecord::Base
  belongs_to :parent
end

Build and save your object parents_controller.rb

class ParentsController < ApplicationController

   def new
     @parent = Parent.new
     @parent.children.build

     respond_to do |format|
       format.html # new.html.erb
       format.json { render json: @parent }
     end
   end

   def create
      #your params should look like.
      params = { 
        parent: {
          name: 'dummy parent', 
          children_attributes: [
            { title: 'dummy child 1' },
            { title: 'dummy child 2' }
          ]
        }
      }

      #You can save your object at once.
      @parent = Parent.create(params[:parent])

      #Or you can set object attributes manually and then save it.
      @parent.name = params[:parent][:name]
      @parent.children_attributes = params[:parent][:children_attributes]
      @parent.save
   end  

end

For more info: http://api.rubyonrails.org/classes/ActiveRecord/NestedAttributes/ClassMethods.html