Rails 3: Get Count of Models via Relationship

In Ruby on Rails (3) with PostgreSQL and given the following three hierarchical models:

class Category
    has_many :posts
end

class Post
    belongs_to :category
    has_many :comments
end

class Comment
    belongs_to :post
end

Is there a way to get the total comment count for each category without having to iterate over each of the posts?

Any help would be appreciated, thank you!