问题
How do polymorphic associations work in Rails? What are their advantages? Is there a way to add belongs_to method just by running a migration?
回答1:
Ryan has a railscast about this that is pretty good.
Belongs_to isn't something you add to a migration, you add it to the model. In the migration, you have to add the foreign key column. For example if you have a post model that belongs to a user, you'd add the user_id column to the post activerecord in a migration. Then you add
belongs_to :user
in the post model separately. Then rails will do its magic in the background to give you the proxy collections in the user model.
来源：https://stackoverflow.com/questions/697840/polymorphic-associations-in-rails