active record

gistfile1.rb

class Person < ActiveRecord::Base
  belongs_to :location
  belongs_to :role
end

class Role < ActiveRecord::Base
  has_many :people
end

class Location < ActiveRecord::Base
  has_many :people
end

gistfile2.rb

Location.joins(:people)
Location.joins(people: :role)
Location.joins(people: :role).where(roles: { billable: true })
Location.joins(people: :role).where(roles: { billable: true }).distinct
Location.joins(:region).merge(Region.order(:name)).order(:name)
Location.from(Location.billable, :locations)

class Location < ActiveRecord::Base
  def self.billable
    joins(people: :role).where(roles: { billable: true }).distinct
  end

  def self.by_region_and_location_name
    joins(:region).merge(Region.order(:name)).order(:name)
  end
end


Location.from(Location.billable, :locations).by_region_and_location_name

gistfile3.rb

class Person < ActiveRecord::Base
  belongs_to :manager, class_name: "Person", foreign_key: :manager_id
  has_many :employees, class_name: "Person", foreign_key: :manager_id
end

Person.
  joins(:manager).
  where.
  not(managers_people: { id: Person.find_by!(name: "Eve") })