多态关联 [英] Polymorphic association

问题描述

如果您具有多态的belongs_to关联，则引用将添加所需的两列:

If you have polymorphic belongs_to associations then references will add both of the columns required:

create_table :products do |t|
  t.references :attachment, :polymorphic => {:default => 'Photo'}
end

将添加一个attachment_id列和一个字符串attachment_type列，其默认值为照片".

will add an attachment_id column and a string attachment_type column with a default value of ‘Photo’.

这到底是什么意思?

推荐答案

基本上，从定义上讲，多态关联增加了创建与许多其他Rails ActiveRecord模型关联的能力.

Basically, polymorphic association by definition adds ability to create associations with many other Rails ActiveRecord Models.

Rails的两列都知道关联所引用的Model，您拥有attachment_type列(即String)(默认值为"Photo"，表名:db中的photos)， 而attachment_id类似于该特定模型/表(即照片)的外键.

Both Columns are there for Rails to know which Model the association is referring to, you have the attachment_type column (ie String) (default value is "Photo",table-name: photos in the db), and the attachment_id is sort of like a foreign key to that particular Model/table (ie Photo).

通常，这为您提供了将一个模型与许多其他模型关联的灵活性.

Usually this provides you flexibility to associate one Model to many others.

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