Rails Parent -> Child creation

rails_inline.rb

require 'bundler/inline'

gemfile(true) do
  source 'https://rubygems.org'

  gem 'rails', '6.0.3'
  gem 'pg'
  gem 'pry'
end

require 'active_record'
require 'minitest/autorun'
require 'logger'

ActiveRecord::Base.establish_connection(adapter: 'postgresql', database: 'test_test_test')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :sequences, force: true do |t|
    t.string :name
    # ...
  end

  create_table :frames, force: true do |t|
    t.references :sequence, null: false
    t.string :url
  end

end

class Sequence < ActiveRecord::Base
  has_many :frames, dependent: :destroy
end

class Frame < ActiveRecord::Base
  belongs_to :sequence
  validates_presence_of :url
end

class SequenceIsCreatedWithFrames < ActiveSupport::TestCase
  def setup
    @sequence = Sequence.new(name: 'Foo')
    @frame = @sequence.frames.build(url: 'http://example.com')

    @sequence.save!
  end

  def test_models_are_persisted
    assert_equal true, @sequence.persisted?
    assert_equal true, @frame.persisted?
  end

  def test_frame_belongs_to_sequence
    assert_includes @sequence.frames, @frame
  end
end

class SequenceWithErrorsInFramesIsNotSaved < ActiveSupport::TestCase
  def setup
    @sequence = Sequence.new(name: 'Foo')
    @frame1 = @sequence.frames.build(url: 'http://example.com')
    @frame2 = @sequence.frames.build

    @sequence.save
  end

  def test_models_are_persisted
    assert_equal false, @sequence.persisted?
  end

  def test_frames_have_errors
    assert_includes @sequence.errors.full_messages, 'Frames is invalid'
    # @sequence.frames.map(&:errors)
    # => [#<ActiveModel::Errors:0x00007f90aaa06dc8 @base=#<Frame:0x00007f908aa89680 id: nil, sequence_id: nil, url: "http://example.com">, @details={}, @messages={}>,
    #<ActiveModel::Errors:0x00007f90aaa05ae0 @base=#<Frame:0x00007f90aaa17768 id: nil, sequence_id: nil, url: nil>, @details={:url=>[{:error=>:blank}]}, @messages={:url=>["can't be blank"]}>]
  end
end