Rails model joined has_many

Question

Here is what I'm trying to do:

class Cashflow < ActiveRecord::Base
    belongs_to from_account, :class_name => 'Account'
    belongs_to to_account, :class_name => 'Account'
end

class Account < ActiveRecord::Base
    has_many :cashflows
end

where Account::cashflows is obviously a list of all cashflows that either have the account_id stored in from_account or in to_account.

I'm confused. What is the proper way of handling such a case? How bad design is this? What would be the proper way of designing such a relation?

Answer 1

I think you have the right structure as there can only two accounts be involved in a particular transaction/cashflow. if you use many to many association you would need to handle the validation for not involving more or less than 2 accounts. For your current structure you can change your moidel associations to be:

class Cashflow < ActiveRecord::Base
  belongs_to from_account, :class_name => 'Account', :foreign_key => :from_account
  belongs_to to_account, :class_name => 'Account', :foreign_key => :to_account
end

class Account < ActiveRecord::Base
  has_many :debits, :class_name => 'Cashflow', :foreign_key => :from_account
  has_many :credits, :class_name => 'Cashflow', :foreign_key => :to_account

  def cashflows
    transactions = []
    transactions << self.debits
    transactions << self.credits
    transactions.flatten!

    ## or may be the following commented way
    # Cashflow.where('from_account = ? OR to_account = ?', self.id, self.id)
  end
end

This way you can keep track of the amount debited/credited in a particular account and also get the accounts involved in a particular transaction/cashflow.