Defining primary and foreign keys in Active Record associations

In one of my Rails 4 projects, I found that it was easier to use the source database’s primary and foreign keys. The source database would drop and reuse keys without leaving a history of changes. Fortunately, Active Record makes it easy to change the keys used by the associations.

has_many and has_one

class Customer < ActiveRecord::Base
  has_many :accounts, primary_key: "customer_number", 
    foreign_key: "customer_number"
  
  has_many :account_sales, through: :accounts
  
  has_one  :main_hq, primary_key: "customer_number", 
    foreign_key: "customer_number", class_name: "Office"
end

The Customer table’s primary key is customer_number. The associated table, accounts, has a column called customer_number that serves as the foreign key.

The same logic goes for the main_hq association. The option class_name is used because the model cannot be derived from the association’s name.

belongs_to

class Account < ActiveRecord::Base
  belongs_to :customer, primary_key: "customer_number", foreign_key: "customer_number"
  has_many :account_sales 

On the Account side, the primary_key refers to the Customer table’s primary key, and the foreign_key is the local attribute or column.

Indexing

Index your tables on the primary and foreign keys.

class AddIndexToCustomers < ActiveRecord::Migration
  def change
    add_index :customers, :customer_number
  end
end

class AddIndexToAccounts < ActiveRecord::Migration
  def change
    add_index :accounts, :customer_number
  end
end