为什么链接模型查询/分配在Rails控制台中不起作用？

Question

How come this works in the rails console:

foo = Ad.find(2)
foo.user = User.find(1)
foo.user # => #<User id: 1, name: "john">

But this doesn't?

Ad.find(2).user = User.find(1)
Ad.find(2).user  # => nil

Answer 1

Because each time you write Ad.find(2) it is returning a new instance of the Ad class whose ID is 2, and your code is changing the associated User on that instance but never saving the change . So in this line:

Ad.find(2).user = User.find(1)

you fetch an instance of Ad with ID 2, set that instance's user association to User.find(1) , but this change is never saved to the database, and is lost once the statement ends. In the next line:

Ad.find(2).user  # => nil

you are just fetching another instance of Ad with ID 2, but since the previous change was never persisted to the DB, user is nil .

Like you showed in the first code snippet, you must use a local variable to temporarily keep a reference to your Ad instance in order to call .save on it, in order to persist to the DB. This should work:

foo = Ad.find(2)
foo.user = User.find(1)
foo.save
Ad.find(2).user  # => #<User id: 1>