console_commands.rb

load "test/fixtures.rb"
customer = Customer.first
# => #<Customer id: 1, name: "Acme Inc.", created_at: "2009-05-24 17:05:45", updated_at: "2009-05-24 17:05:45">
customer.projects
# => [#<Project id: 1, name: "Rails", created_at: "2009-05-24 17:05:45", updated_at: "2009-05-24 17:05:45">]
customer.projects.destroy_all
# => []
Project.all
# => []

contact.rb

class Contact < ActiveRecord::Base
  belongs_to :customer
  belongs_to :project
end

customer.rb

class Customer < ActiveRecord::Base
  has_many :contacts
  has_many :projects, :through => :contacts
end

fixtures.rb

customer = Customer.create! :name => "Acme Inc."
project = Project.create! :name => "Rails"
contact = Contact.create! :name => "Bugs Bunny"
project.contacts << contact
customer.contacts << contact

project.rb

class Project < ActiveRecord::Base
  has_many :contacts
end