在Rails中查找具有多个外键值的记录

Question

I've got two models User and Post where a :user has_many :posts

How do I find a list of users who have more than n posts?

I'm looking for a simple statement like where rather than something to do with using scopes.

Answer 1

The where query does not allow aggregate functions like count inside it, the ideal method to tackle this problem is using the having method.

User.joins(:posts).group('users.id').having('COUNT(*) >= ?', n)

This will be mapped to

SELECT "users".* FROM "users" INNER JOIN "posts" ON "posts"."user_id" = "users"."id" GROUP BY users.id HAVING COUNT(*) >= n

There is another method possible which is more easier, by using counter cache inside the Post Model and keeping the count as a posts_count column on the user table.

belongs_to :user, counter_cache:true 
User.where('posts_count > ?', n)

But I prefer the first method as it does not involve any DB changes and is pretty straight forward

Answer 2

User.join(:posts).group('users.id').having('count(user_id) > n')

Another way of solving this is to use counter_cache and add a posts_count column on the user table.

belongs_to :user, counter_cache:true # Inside the Post model

Then you can do

User.where('posts_count > ?', n)

Answer 3

Let try this:

User.where('id IN (SELECT user_id 
                   FROM posts
                   GROUP BY user_id
                   HAVING COUNT(*) > ?
                  )', n)