Rails模型中2个表之间的关系如何？

Question

I have 2 tables in Rails:

CURRENCIES:  
-------------  
| id | name |  
-------------  
|  1 | USD  |  
|  2 | EUR  |   
-------------

ITEMS:  
----------------------------------------  
| id | name      | price | currency_id | 
----------------------------------------  
|  1 | product a |   100 |           1 |
|  2 | product b |    20 |           2 |
|  3 | product c |    60 |           2 |
|  4 | product d |  NULL |        NULL |
----------------------------------------

ITEMS.price and ITEMS.currency_id are allow NULL , but ITEMS.currency_id must be set when ITEMS.price != NULL .

How does the relationship between those tables?

Answer 1

This is how you should do the one-to-one relationship in RoR:

class Item < ActiveRecord::Base
  has_one :currency
end

Depending on your DBMS, you could add a check constraint so that price and currency_id must be either both null or both with a value.

It also seems to me you could put that in another table

PRICES
---------------------------
| id | value | currency_id |  
---------------------------
|  1 | 100   |           1 | 
|  2 | 200   |           2 | 
---------------------------