wat.rb

require 'active_record'
require 'minitest/autorun'

ActiveRecord::Base.establish_connection adapter: 'sqlite3', database: ':memory:'
ActiveRecord::Base.connection.instance_eval do
  create_table "timesheets"

  create_table "projects_timesheets" do |t|
    t.integer  "project_id"
    t.integer  "timesheet_id"
  end

  create_table "projects"
end

class Project < ActiveRecord::Base
  has_many :projects_timesheets
end

class ProjectsTimesheet < ActiveRecord::Base
  belongs_to :project
  belongs_to :timesheet
end

class Timesheet < ActiveRecord::Base
  has_many :projects_timesheets
  has_many :projects, :through => :projects_timesheets

  accepts_nested_attributes_for :projects_timesheets
end

class MyTest < Minitest::Test
  def test_has_timesheet
    timesheet = Timesheet.new.tap do |timesheet|
      timesheet.projects += [Project.new]
    end
    assert_equal timesheet, timesheet.projects_timesheets.first.timesheet
  end
end