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Question

I am making a dating app similar to tindler, where users can like or dislike each other. If two users both like each other, they should be able to chat with each other. I have come up with the following query to handle pulling a list of users you can chat/have chats with already - the problem i'm having is i only want to pull the most recent chat message, solely to display a little blurb before you click into the chat itself. My query works, but it returns the oldest (lowest ID) chat record, instead of the newest. Order by does not seem to have an impact on returning the correct result.

$data = $this->db->select('users.id,display_name,city,state,gender,users_pictures.picture,users_chats.message')
                ->join('users_pictures','users_pictures.user_id=users.id')
                ->join('users_chats','users_chats.user_id=users.id OR users_chats.foreign_user_id=users.id','left outer')
                ->where('EXISTS (SELECT 1 FROM users_likes_dislikes ld WHERE (ld.foreign_user_id = '.$this->user_id.' AND ld.user_id=users.id AND ld.event_type=1) OR (SELECT 1 FROM users_likes_dislikes ld WHERE ld.foreign_user_id = users.id AND ld.user_id='.$this->user_id.' AND ld.event_type=1))', '', FALSE)
                ->where('NOT EXISTS (SELECT 1 FROM users_blocks ub WHERE (ub.foreign_user_id = users.id AND ub.user_id='.$this->user_id.') OR (SELECT 1 FROM users_blocks ub WHERE ub.foreign_user_id = '.$this->user_id.' AND ub.user_id=users.id))', '', FALSE)
                ->where('((users_chats.user_id='.$this->user_id.' OR users_chats.foreign_user_id='.$this->user_id.') OR (users_chats.user_id is NULL AND users_chats.foreign_user_id is NULL))')
                ->order_by('users_chats.id','DESC')
                ->group_by('users.id')              
                ->get('users')
                ->result_array();

Here is the current mysql table for users_chats:

id user_id foreign_user_id message created
1 1 4 test 2013-05-22 15:42:44
2 1 4 test2 2013-05-22 15:44:38

I assumed the order_by would ensure that the test2 message is what displayed.

Here is sample output:

Array ( [0] => Array ( [id] => 4 [display_name] => testinguser [city] => west hills [state] => ca [gender] => 2 [picture] => testasdfasdf.jpg [message] => test ) )

Any help is much appreciated :)

edit - the query itself (without group by, this works but i need it to group to the user.id so that i don't have multiple entries for the same user in the array):

SELECT
  `users`.`id`,
  `display_name`,
  `city`,
  `state`,
  `gender`,
  `users_pictures`.`picture`,
  `users_chats`.`message`
FROM (`users`)
  JOIN `users_pictures`
    ON `users_pictures`.`user_id` = `users`.`id`
  JOIN `users_chats`
    ON `users_chats`.`user_id` = `users`.`id`
       OR users_chats.foreign_user_id = users.id
WHERE EXISTS(SELECT
           1
         FROM users_likes_dislikes ld
         WHERE (ld.foreign_user_id = 1
            AND ld.user_id = users.id
            AND ld.event_type = 1)
          OR (SELECT
            1
              FROM users_likes_dislikes ld
              WHERE ld.foreign_user_id = users.id
              AND ld.user_id = 1
              AND ld.event_type = 1))
    AND NOT EXISTS(SELECT
             1
           FROM users_blocks ub
           WHERE (ub.foreign_user_id = users.id
              AND ub.user_id = 1)
            OR (SELECT
                  1
                FROM users_blocks ub
                WHERE ub.foreign_user_id = 1
                AND ub.user_id = users.id))
    AND ((users_chats.user_id = 1
       OR users_chats.foreign_user_id = 1)
      OR (users_chats.user_id is NULL
          AND users_chats.foreign_user_id is NULL))
ORDER BY `users_chats`.`created` DESC

Answer 1

Your group by clause could be the culprit here. I believe that the grouping operation happens first, leaving you with your first result. Rather than selecting all these rows (when there's tons it'll take longer), you should specify how many you want - it looks to me like that isn't too far out of the picture here, anyways. Specify how many you want, get rid of the group by clause, and you should order by date since you HAVE a date column. Does that help?

Answer 2

I'm not sure how to do it with your db Abstraction, but the query you want is

SELECT
   `users`.`id`,
   `display_name`,
   `city`,
   `state`,
   `gender`,
   `users_pictures`.`picture`,
   chats1.`message`
  FROM (`users`)
    JOIN `users_pictures`
      ON `users_pictures`.`user_id` = `users`.`id`
    JOIN `users_chats` AS chats1
      ON (chats1.`user_id` = `users`.`id`
         OR chats1.foreign_user_id = users.id)

Here comes the important part

         AND NOT EXISTS(
           SELECT *
             FROM users_chats AS chats2
             WHERE ((chats2.user_id = chats1.user_id AND chats2.foreign_user_id = chats1.foreign_user_id)
               OR (chats2.user_id = chats1.foreign_user_id AND chats1.user_id = chats2.foreign_user_id))
               AND chats2.created_date > chats1.created_date --which I assume is a time stamp
           )

It's not pretty, I know.

  WHERE EXISTS(SELECT 1
    FROM users_likes_dislikes ld
      WHERE (ld.foreign_user_id = 1
        AND ld.user_id = users.id
        AND ld.event_type = 1)
        OR (SELECT 1
          FROM users_likes_dislikes ld
          WHERE ld.foreign_user_id = users.id
          AND ld.user_id = 1
          AND ld.event_type = 1)
    )
    AND NOT EXISTS(SELECT 1
       FROM users_blocks ub
       WHERE (ub.foreign_user_id = users.id
          AND ub.user_id = 1)
        OR (SELECT
              1
            FROM users_blocks ub
            WHERE ub.foreign_user_id = 1
            AND ub.user_id = users.id)
    )
    AND ((chats1.user_id = 1
        OR chats1.foreign_user_id = 1)
      OR (chats1.user_id is NULL
        AND chats1.foreign_user_id is NULL))
  ORDER BY `users_chats`.`created` DESC

Basically, only successfully join if there's no more recent message. There are some better native solutions - TSQL (Microsoft SQL Server) has CROSS APPLY, which would be great here - but without knowing more about your DB layer, I can't be sure. You may want to considered re-architecting your chat structure:

Users(int id /*also other user info*/)
chats(int id, datetime date_initiated, bool /*or bit, or short int*/ is_active)
chat_users (int chat_id, int user_id)
chat_messages (int chat_id, int user_id /*author*/, datetime date_sent, varchar(n) message)

With a structure like that, you could get all your most recent messages like this:

SELECT *
  FROM Users AS u
    INNER JOIN chat_users AS cu
      ON u.id = cu.user_id
    INNER JOIN chats AS c
      ON c.id = cu.chat_id
        AND c.is_active = 1
    INNER JOIN chat_messages AS m
      ON m.chat_id = c.id
        AND NOT EXISTS (
          SELECT 1
            FROM chat_messages AS m2
              WHERE m2.chat_id = m.chat_id
                AND m.date_sent < m2.date_sent
        )
     INNER JOIN Users as sender
       ON m.user_id = sender.id
  WHERE u.id = ###
  ORDER BY m.date_sent DESC

You could even create a "Chat most recent message" view like:

CREATE VIEW Chat_Recent AS
  SELECT * /* WHATEVER YOU LIKE */ 
    FROM chats AS c
      INNER JOIN chat_messages AS m
        ON m.chat_id = c.id
          AND NOT EXISTS (
            SELECT 1
              FROM chat_messages AS m2
                WHERE m2.chat_id = m.chat_id
                  AND m.date_sent < m2.date_sent
          )
       INNER JOIN Users as sender
         ON m.user_id = sender.id

Answer 3

Try MySQL MAX():

$this->db->join('(SELECT MAX(message) AS lastMsg FROM users_chats WHERE users_chats.user_id=users.id OR users_chats.foreign_user_id=users.id GROUP BY users.id)', 'left outer');

Then add the "lastMsg" on your select.