Question

The Rails guides state that a scope can be called on an association. But then further on, it states that the scoped method, which returns an ActiveRecord::Relation object, “may come in handy…on associations”. If a scope can be called on an association, what additional functionality does scoped provide?

Answer 1

scoped returns an anonymous scope. From the API docs:

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

Here’s the example:

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?