for Ari BostonRB question https://groups.google.com/forum/?fromgroups=#!topic/boston-rubygroup/g5SnKxxdffo

models.rb

require 'active_record'

class Grouping < ActiveRecord::Base
  has_and_belongs_to_many :values
  has_and_belongs_to_many :characteristics
end

class Characteristic < ActiveRecord::Base
  has_and_belongs_to_many :groupings
  has_many :values, :through => :groupings
end

class Value < ActiveRecord::Base
  has_and_belongs_to_many :groupings
  has_many :characteristics, :through => :groupings
end


ActiveRecord::Base.establish_connection(
  database: "ari",
  adapter: "postgresql"
)

Grouping.delete_all
Characteristic.delete_all
Value.delete_all
g1 = Grouping.create name: "group g1"
v1 = Value.create name: "value v1"
c1 = Characteristic.create name: "characteristic c1"
c2 = Characteristic.create name: "characteristic c2"
g1.characteristics << c1 << c2
g1.values << v1

puts "g1: ", g1.inspect
puts "g1 characteristics: ", g1.characteristics.map(&:name).inspect
puts "c1 values: ", c1.values.map(&:name).inspect
puts "v1 characteristics: ", v1.characteristics.map(&:name).inspect

__END__


Output will be like

g1: 
#<Grouping id: 7, name: "group g1">
g1 characteristics: 
["characteristic c1", "characteristic c2"]
c1 values: 
["value v1"]
v1 characteristics: 
["characteristic c1", "characteristic c2"]

schema.sql

create table characteristics (
  id serial primary key,
  name varchar
);

create table groupings (
  id serial primary key,
  name varchar
);

create table values (
  id serial primary key,
  name varchar
);

create table groupings_values  (
  grouping_id integer references groupings(id) on delete cascade,
  value_id integer references values(id) on delete cascade
);
create table characteristics_groupings  (
  characteristic_id integer references characteristics(id) on delete cascade,
  grouping_id integer references groupings(id) on delete cascade
);