association

use class methods( has_one, belongs_to, has_many,  has_and_belongs_to_many)

to create instance methods

has_one & belongs_to

has_many & belongs_to

has_and_belongs_to_many

the parameter for these methods:

:dependent => true

for delete.  

ex:

class Customer <>

         has_many  :orders,

                             :dependent=>true

end

if the customer A is deleted, its orders are also deleted

:order

ex:

:order=>"created_at ASC"

ascending order according to created_at field