Rails 组按用户上次创建的帖子或上次创建的评论排序

Question

I have the following relationship, a discussion which has_many posts which has_many comments

I am trying to implement a personalised user dashboard which orders lastest discussions depending on the user very last change on a particular discussion, so if a user makes a post on a discussion, this discussion should be on the top of the dashboard and if a user makes a comment on any of the posts in a given discussion, this discussion should also now be on the top of his dashboard.

Until now i can only get them to work separately, so i am struggling to combine them.

Only order by last Post:

@discussions = Discussion.joins(:posts).where(posts: {user: current_user}).group('discussions.id').order('MAX(posts.created_at) DESC')

Only order by last Comment:

@discussions = Discussion.joins(:comments).where(comments: {user: current_user}).group('discussions.id').order('MAX(comments.created_at) DESC')

My Attempt to combine them which doesn't work:

@discussions = Discussion.joins(:posts,:comments).where(posts: {user: current_user}).where(comments: {user: current_user}).group('discussions.id').order('MAX(posts.created_at) DESC', 'MAX(comments.created_at) DESC')

This only works depending on what i write on the .order but both never work..

Any help will be much appreciated or any other way of achieving the purpose is also welcome!

Update:

A kind user helped me get a bit further but still not quite the results i am looking for

@discussions = Discussion.joins(:posts,:comments).where(posts: {user: current_user}).where(comments: {user: current_user}).group('discussions.id').order('GREATEST(MAX(posts.created_at), MAX(comments.created_at)) DESC')

The problem here is, that it only works if both a post and a comment exist within a discussion, but i want it to be if either is created the discussion list should be updated with the most recent changed discussion on top.

Answer 1

query = """
SELECT discussions.*, MAX(sub2.MostRecentDate)
FROM discussions INNER JOIN (
  SELECT posts.id, posts.discussion_id,
    CASE
      WHEN sub1.MostRecentCommentDate IS NULL OR posts.updated_at >= sub1.MostRecentCommentDate THEN posts.updated_at
      ELSE sub1.MostRecentCommentDate
    END AS MostRecentDate
  FROM posts LEFT OUTER JOIN (
    SELECT comments.post_id, MAX(comments.updated_at) AS MostRecentCommentDate
    FROM comments
    WHERE comments.user_id = ?
    GROUP BY comments.post_id
  ) AS sub1
  ON sub1.post_id = posts.id AND posts.user_id = ?
) AS sub2
ON discussions.id = sub2.discussion_id
GROUP BY discussions.id
ORDER BY MAX(sub2.MostRecentDate) DESC
"""

Discussion.find_by_sql(Discussion.send(:sanitize_sql_array, [query, current_user.id, current_user.id]))

First, get maximum of updated_at on comments table grouped by post_id

SELECT comments.post_id, MAX(comments.updated_at) AS MostRecentCommentDate
FROM comments
GROUP BY comments.post_id AS sub1

Result:

|post_id|MostRecentCommentDate|
|-------|---------------------|
|  ...  |        ....         |

Next, left joins posts table with the result table above and compare the sub1.MostRecentCommentDate with posts.updated_at to get the final MostRecentDate

SELECT posts.id, posts.discussion_id,
  CASE
    WHEN sub1.MostRecentCommentDate IS NULL OR posts.updated_at >= sub1.MostRecentCommentDate THEN posts.updated_at
    ELSE sub1.MostRecentCommentDate
  END AS MostRecentDate
FROM posts LEFT OUTER JOIN (
...
) AS sub2

Result:

|discussion_id|MostRecentDate|
|-------------|--------------|
|     ...     |     ....     |

Finally, joins the discussion table with result table sub2 .

SELECT discussions.*, MAX(sub2.MostRecentDate) 
FROM discussions 
INNER JOIN (
...
) AS sub2
ON discussions.id = sub2.discussion_id
GROUP BY discussions.id
ORDER BY MAX(sub2.MostRecentDate) DESC

Will give us a table of discussions order by sub2.MostRecentDate

Note:

1. The return value of find_by_sql() is an array not an ActiveRecord_Relation .

2. The reason of calling sanitize_sql_array is to safely pass parameter to the raw query. Reference here