Question

Imagine a simple I-O-U app where there are a User model and a Transaction model.

The User table has the following columns:

• id
• name

The Transaction table has the following columns:

• lender_id (foreign key to User.id)
• borrower_id (foreign key to User.id)
• date

A user can only lend or borrow to another user that exists in the User table.

An example would be:

User:

 id | name
-------------
  1 | name A
  2 | name B
  3 | name C

Transaction:

 lender_id | borrower_id |  date
----------------------------------
     1     |     2       |  date1
     1     |     3       |  date2
     3     |     2       |  date3

How would I implement the associations in the model classes?

Answer 1

On transaction:

belongs_to :lender, :class_name => "User"
belongs_to :borrower, :class_name => "User"

On user:

has_many :loans, :class_name => "Transaction", :foreign_key => "lender_id"
has_many :debts, :class_name => "Transaction", :foreign_key => "borrower_id"

This gives you:

u = User.first
u.loans # [transaction, transaction, ...]
u.debts # [transaction, transaction, ...]

And

t = Transaction.first
t.lender # User
t.borrower # User

Hope this helps!