ruby on rails：为什么没有为每个新模型关联进行迁移？

Question

My understanding is that relationships between tables are implemented through foreign keys. But when you add an association between models (belongs_to, etc), there is no change in the database schema. Why is that? How does rails actually know there is an association or relationship?

Answer 1

The trick here is that belongs_to, has_one, has_many only works if the appropriate keys are already in the database.

The keys are usually <model>_id .

So a belongs_to relationship between post and blog requires you to have a blog_id in the posts table that you had to create using a relation.

Same goes for a has_many between blog and post.

class Blog < ActiveRecord::Base
  has_many :posts
end

Requires a migration that creates the blog_id foreign key in the posts table.

Answer 2

Rails will recognize the associations among the all tables by mentioning those relations in the models only, but not only with the foreign keys. If you don't follow the naming convention for the foreign keys, rails will loose finding the relation between the tables even you mention the relation in the models. Then you have to tell the foreign_key, and class names explicitly.

class Article < ActiveRecord::Base
 has_many :comments
end 

class Comment < ActiveRecord::Base
 belongs_to :article
end

In the comments table, you have to maintain the article_id along with its own fields.
Then rails will automatically assumes that article_id as foriegn_key .
If you write something else(Ex: art_id ) rather article_id you have to tell as:

class Comment < ActiveRecord::Base
 belongs_to :article, :class_name=>"Article", :foreign_key=>:art_id
end