Rails: for 'a belongs_to b', a.b.id != a.b_id

gistfile1.rb

irb(main):010:0> t = Tag.new(:rfid => 999, :item_id => 1)
=> #<Tag id: nil, rfid: 999, created_at: nil, updated_at: nil, item_id: 1>

irb(main):011:0> t.item
=> #<Item id: 1, description: "burger", price: #<BigDecimal:b6f6f46c,'0.5E1',4(8)>, created_at: "2011-11-12 03:16:19", updated_at: "2011-11-12 03:16:19">

irb(main):012:0> t.item_id = 2
=> 2

irb(main):013:0> t.save
=> true

irb(main):016:0> t
=> #<Tag id: 1, rfid: 999, created_at: "2011-11-12 22:42:49", updated_at: "2011-11-12 22:42:49", item_id: 2>

irb(main):017:0> t.item
=> #<Item id: 1, description: "burger", price: #<BigDecimal:b6f35fa0,'0.5E1',4(8)>, created_at: "2011-11-12 03:16:19", updated_at: "2011-11-12 03:16:19">

irb(main):018:0> t.item.id == t.item_id
=> false

You have to reload the association after saving the Tag.

ruby-1.9.2-p290 :004 > t = Tag.new(:rfid => 999, :item_id => 1)
=> #<Tag id: nil, rfid: 999, item_id: 1, created_at: nil, updated_at: nil>
ruby-1.9.2-p290 :005 > t.item
Item Load (0.1ms) SELECT "items".* FROM "items" WHERE "items"."id" = 1 LIMIT 1
=> #<Item id: 1, description: "burger", price: #BigDecimal:7fb974c38d60,'0.3599E2',18(18), created_at: "2011-11-13 05:11:05", updated_at: "2011-11-13 05:11:05">
ruby-1.9.2-p290 :006 > t.item_id = 2
=> 2
ruby-1.9.2-p290 :007 > t.save
SQL (8.9ms) INSERT INTO "tags" ("created_at", "item_id", "rfid", "updated_at") VALUES (?, ?, ?, ?) [["created_at", Sun, 13 Nov 2011 05:17:29 UTC +00:00], ["item_id", 2], ["rfid", 999], ["updated_at", Sun, 13 Nov 2011 05:17:29 UTC +00:00]]
=> true
ruby-1.9.2-p290 :008 > t
=> #<Tag id: 2, rfid: 999, item_id: 2, created_at: "2011-11-13 05:17:29", updated_at: "2011-11-13 05:17:29">
ruby-1.9.2-p290 :009 > t.reload
Tag Load (0.1ms) SELECT "tags".* FROM "tags" WHERE "tags"."id" = ? LIMIT 1 [["id", 2]]
=> #<Tag id: 2, rfid: 999, item_id: 2, created_at: "2011-11-13 05:17:29", updated_at: "2011-11-13 05:17:29">
ruby-1.9.2-p290 :010 > t.item
Item Load (0.1ms) SELECT "items".* FROM "items" WHERE "items"."id" = 2 LIMIT 1
=> #<Item id: 2, description: "burger", price: #BigDecimal:7fb973915aa8,'0.3599E2',18(18), created_at: "2011-11-13 05:12:02", updated_at: "2011-11-13 05:12:02">
ruby-1.9.2-p290 :011 > t.item.id == t.item_id
=> true
ruby-1.9.2-p290 :012 >