ruby-on-rails-3 - 为什么 Rails 指南建议在关联上使用 scoped 方法？

Question

Rails guides声明可以在关联上调用范围。但是然后 further on ，它声明 scoped 方法返回一个 ActiveRecord::Relation 对象，“可能会派上用场......在关联上”。如果可以在关联上调用作用域，scoped 提供哪些附加功能？

Answer 1

scoped返回一个匿名 范围。来自 API 文档:

Anonymous scopes tend to be useful when procedurally generating complex queries, where passing intermediate values (scopes) around as first-class objects is convenient.

例子如下:

posts = Post.scoped
posts.size # Fires "select count(*) from  posts" and returns the count
posts.each {|p| puts p.name } # Fires "select * from posts" and loads post objects

fruits = Fruit.scoped
fruits = fruits.where(:color => 'red') if options[:red_only]
fruits = fruits.limit(10) if limited?