Core Data relationship between existing entities

I have two tables of data. One is table_A(id, x, x, b_id) and table_B(id, x). I would like to add a relationship between b_id from table_A to id of table_B. I already have a JSON data like that, and I tried with Xcode to make so connection, but all I can make is a new relationship between those two.

I'm new to this, so would apreciate any help.