add with_association_target to ActiveRecord Relation class

monkey_patch.rb

ActiveRecord::Relation.class_eval do
  def with_association_target(targets)
    @association_target = (@association_target||{}).merge(targets)
    self
  end

  def load_with_association_targets(*args)
    load_without_association_targets(*args).tap do
      @records.each{|record| set_association_targets(record) }
    end
  end
  alias_method_chain :load, :association_targets

  private

  def set_association_targets(record)
    if @association_target.present?
      @association_target.each do |association_name, target|
        record.association(association_name).target = target
      end
    end
  end
end

test.rb

require 'test_helper'

class WithAssociationTargetTest < ActiveSupport::TestCase
  fixtures :all

  def setup
    @hermine_ramon = clients(:hermine_ramon)
    @new_client    = clients(:new_client)
    @association = @hermine_ramon.tasks
  end

  test "association with scope" do
    assert_equal @new_client, @association.order('id').with_association_target(client: @new_client)[0].client
  end

  test "first" do
    assert_equal @new_client, @association.order('created_at').with_association_target(client: @new_client).first.client
  end

  test "multiple" do
    new_task_template = TaskTemplate.new
    subject = @association.order('id') \
      .with_association_target(client: @new_client, task_template: new_task_template)
    assert_equal @new_client, subject[0].client
    assert_equal new_task_template, subject[0].task_template
  end

  test "chains" do
    new_task_template = TaskTemplate.new
    subject = @association.order('id') \
      .with_association_target(client: @new_client) \
      .with_association_target(task_template: new_task_template)
    assert_equal @new_client, subject[0].client
    assert_equal new_task_template, subject[0].task_template
  end
end