'拆分'ActiveRecord系列

Question

假设我有两个模型Post和Category:

class Post < ActiveRecord::Base
  belongs_to :category
end

class Category < ActiveRecord::Base
  has_many :posts
end

有没有一种方法可以让我做类似的事情

posts = Post.find(:all)

p = Array.new

p[1] = posts.with_category_id(1)
p[2] = posts.with_category_id(2)
p[3] = posts.with_category_id(3)
...

or

p = posts.split_by_category_ids(1,2,3)

=> [posts_with_category_id_1, 
    posts_with_category_id_2,
    posts_with_category_id_3]

换句话说,通过选定的类别ID将所有帖子的集合"拆分"为数组

Answer 1

尝试上课时的group_by功能Array:

posts.group_by(&:category_id)

有关更多详细信息,请参阅API 文档.

警告:

当潜在数据集很大时,不应在Ruby代码中执行分组.group_by当最大可能的数据集大小<1000时,我使用该函数.在您的情况下,您可能有1000个Posts.处理这样的数组会给你的资源带来压力.依靠数据库来执行分组/排序/聚合等.

这是一种方法(类似的解决方案建议nas)

# returns the categories with at least one post
# the posts associated with the category are pre-fetched
Category.all(:include => :posts, 
    :conditions => "posts.id IS NOT NULL").each do |cat| 
  cat.posts
end