Question

I want to build a query in Rails 3.2 (MYSQL), where I sort a group of posts by the number of your friends that are interacting with it. I can limit this to last 7 days.. My current thinking is:

1. Maintain the following tables / relationships:
   Users has many Friends
   Post has many Interactors

2. __ 🙂 I’m not quite sure how to most efficiently build the query. Perhaps, naively Id’ think I’d first query to get the users friends.. put that into an array.. then query the interactors (last 7 days), where the user_id is ‘in’ my friends list.. in this query, lazy load the posts as well.. then on the front end count the number of interactors per post.. attach that to the posts.. and then sort them? Must be a more efficient way..

Thanks!

Answer 1

Assuming there is a Friend table containing UserID and FriendID, and a PostInteraction table that contains PostID and UserID, you would:

select pi.PostID, count(distinct f.FriendID) as Count
from PostInteraction pi
inner join Friend f on pi.UserID = f.FriendID
where f.UserID = @MyUserID
group by pi.PostID
order by count(distinct f.FriendID) desc

I have assumed the same friend could have multiple interactions with a post and that you only want to count that once, thus the use of DISTINCT in the count.