Question

I would like to identify and count the posts that have a specific number of messages and a post.location that is not nil.

I have approximately 100k rows and it is for a statistical use so I need fast queries (I might have to index post.location by the way).

How can I do it the easiest and quickest way?

Here is my schema :

create_table "posts", :force => true do |t|
  t.string   "ref"
  t.string   "title"
  t.string   "author"
  t.datetime "created_at"
  t.datetime "updated_at"
  t.string   "location"
  t.float    "lat"
  t.float    "long"
end

create_table "messages", :force => true do |t|
  t.integer  "post_id"
  t.integer  "status_id"
  t.integer  "user_id"
  t.string   "content"
  t.datetime "created_at"
  t.datetime "updated_at"
  t.float    "lat"
  t.float    "long"
end

add_index "messages", ["post_id"], :name => "index_messages_on_post_id"
add_index "messages", ["created_at"], :name => "index_messages_on_created_at"

Answer 1

The SQL Query you are looking for should look a lot like this:

SELECT COUNT(posts.id)
FROM posts
JOIN messages ON (posts.id = messages.post_id)
GROUP BY (posts.id) HAVING (count(messages.id) > ?)
WHERE posts.location IS NOT NULL

You can then convert that to a ActiveRecord Query or just use find_by_sql.
(I wrote this from memory so you might have to tweak it in some places .. but the general idea should work).