ActiveRecord Associations: Any gotchas if has_many

问题:

A phone has many messages.

An email address has many messages.

A message either belongs to a phone, email, or neither. The belongs_to association is optional.

The following associations seem to work fine for these relationships:

• Phone model has_many :messages
• Email model has_many :messages
• Message model does NOT have belongs_to :phones, :email

Is this okay or is there some proper way to specify a "can_belong_to" relationship?

回答1:

It is completely correct unidirectional relation. Using both is sometimes called "curcular dependency" by some purists and may cause problems when using validates_associated.

From the other side using only has_many :messages may be not enough when you want retrieve phone information from one message. Generally it is matter of convenience.

回答2:

The model with the belongs_to associations holds the foreign keys (e.g. messages table would have phone_id and email_id columns).

The belongs_to association combined with has_many lets you easily access associated records:

phone.messages
message.phone

So without the belongs_to and FK columns, the has_many association isn't very useful.

It seems like in this case you may want a many-to-many relationship such as has_and_belongs_to_many as a message can have many recipients.