Employee.rb

class Employee < ActiveRecord::Base
  has_many :timeslots
  scope :free_time, lambda {|day| joins(:timeslots).merge(Timeslot.on(day)) }
end

Timeslot.rb

class Timeslot < ActiveRecord::Base
  belongs_to :employee 
  scope :on, lambda { |this_date| where("start >= ? AND end <= ?", this_date, this_date+ 1.day)}
end