Just a small question relating to relationship

[eluser]dhaulagiri[/eluser]
Probably this question shouldn't be here but I am confused how do we establish relationship between two table in CI ? If I have a 'table_A' with id1 and 'table_B' with id1 and id2 , how do i get the value of table_A.id1 in table B ? Probably this is a database question.

I am developing p2p messaging system in CI, just unable to send message to particular person and reply the same person. any suggestion would be great.