how to append a has_many association without hitting the database immediately

Gemfile

source 'https://rubygems.org'

gem 'activerecord', '~> 3.2.18'
gem 'sqlite3', '= 1.3.5'
gem 'rspec'

has_many_soft_append.rb

require "active_record"
ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ":memory:")

RSpec.configure do |config|
  config.before(:suite) do
    Class.new(ActiveRecord::Migration) do
      def change
        create_table :parents
        create_table :children do |t|
          t.references :parent
        end
      end
    end.new.change
  end
end

class Parent < ActiveRecord::Base
  has_many :children, :inverse_of => :parent
end

class Child < ActiveRecord::Base
  belongs_to :parent, :inverse_of => :children
end

describe Child do
  describe "when new_record" do
    let(:parent) { Parent.create! }
    before(:each) { expect(parent).to be_persisted }
    it "writes to database using <<" do
      parent.children << subject
      expect(subject).to be_persisted
      expect(subject.parent_id).to eq(parent.id)
      expect(parent.children).to include(subject)
    end
    it "does not write to database using using add_to_target" do
      parent.association(:children).add_to_target(subject)
      expect(subject).not_to be_persisted
      expect(subject.parent_id).to be_nil
      expect(parent.children).to include(subject)
      parent.save!
      expect(subject).to be_persisted
    end
  end
  describe "when persisted" do
    let(:parent) { Parent.create! }
    subject { described_class.create! }
    before(:each) { expect(parent).to be_persisted }
    before(:each) { expect(subject).to be_persisted }
    it "writes to database using <<" do
      parent.children << subject
      expect(subject).to be_persisted
      expect(subject.parent_id).to eq(parent.id)
      expect(parent.children).to include(subject)
    end
    it "does not write to database using using add_to_target" do
      parent.association(:children).add_to_target(subject)
      expect(subject).to be_persisted # Since it already was persisted
      expect(subject.parent_id).to be_nil # This is less than ideal but will have to do
      expect(parent.children).to include(subject)
      parent.save!
      expect(subject).to be_persisted
    end
  end
end