Join Model

The idea is to use join models to leverage Active Record’s has_many :through() macro. This is taken from Chad Fowler, Rails Recipes, 2012. (Chapter One)

The has_and_belongs_to_many() (habtm) macro is used for join tables, but those join tables (for example a ‘subscriptions’ table joining Magazines and Readers) do not have any data in them other than the relationship. For example, you would name a relationship table magazines_readers, and include only the id’s for Magazine and Reader.

The idea here is to store information in the join table, and it is called a join model. The relationship item has dates.

We will use a very simple notion of a Car, a Driver, and a Lease. The Lease has a Start Date, an End Date, and a Rate.

This example is also serving to explore the minimum number of files which are necessary to run Active Record and Rails.

Fowler calls this join model a first-class concept - a Lease. It exists by itself.

Here are the relationships. A Car has many Leases; but it also has many Drivers. A Driver has many Leases; but it also has many Cars. Each Lease belongs to a Car, and each Lease belongs to a Driver. The relationship between Driver and Car (and vice versa, Car and Driver) is the one using the :through. The relationship between Car and Driver is then implicitly many-to-many.

The interesting things are as follows:

You can do this: car.leases << lease. You have to do lease.save at the end in order to persist the relationships which have been added.

Then, you can do car.drivers or driver.cars. The relation is implicit, and works through the :through parameter of the has_many declarations.

ActiveRecord generates a SQL query like this: SELECT “cars”.* from “cars” inner join “leases” on “cars”.id = “leases”.car_id where ((“leases”.reader_id = 1))

Note that there is an interesting accessor with a SQL condition in the Car model.

This is run by using rails runner app/myscript.rb.

The steps to generating this app were:

1. At the command prompt, create a new Rails application: rails new myapp -J --skip-bundle --skip-test-unit --skip-prototype (where myapp is the application name)

2. Change directory to myapp and edit the Gemfile. Add therubyracer. Remove sqlite3 and replace it with mysql2; do bundle install.

3. Edit the automatically generated database.yml file to use MySQL2 instead of sqlite3.

4. Run rails g model driver name.

5. Run rails g model car make model_name serial.

6. Create the lease db/migrate file:

   class CreateLeases < ActiveRecord::Migration

     def change
       create_table :leases do |t|
         t.integer :driver_id
         t.integer :car_id
         t.date :start_date
         t.date :end_date
         t.string :rate
   
         t.timestamps
       end
     end
   end

7. Run rake db:create.

8. Run rake db:migrate.

9. Run rake db:test:prepare.

10. Add this to the Car model:

class Car < ActiveRecord::Base
  attr_accessible :make, :model_name, :serial
  has_many :leases
  has_many :drivers, :through => :leases
  # named accessor
  has_many :cheap_drivers,
    :through => :leases,
    :source => :driver,
    :conditions => ['rate < 5.0']
end

1. Add this to the Driver model:

   class Driver < ActiveRecord::Base

   attr_accessible :name
   has_many :leases
   has_many :cars, :through => :leases
   

   end

2. Add this to the Lease model:

   class Lease < ActiveRecord::Base

   attr_accessible :start_date, :end_date, :rate
   belongs_to :car
   belongs_to :driver
   

   end

3. Run rake db:seed to create data. That looks like this:

   lease = Lease.create(:start_date => “2012-02-14”, :end_date => “2012-08-14”, :rate => “4.5”) car = Car.create(:make => “Mazda”, :model_name => “GLC”, :serial => “MG1”) driver = Driver.create(:name => “Babe”)

   # Wild syntax car.leases << lease driver.leases << lease

   lease.save

4. Then run rails runner app/myscript.rb.