How to select unique records, make groupBy work after union?

Question

PHP

There is a table (chat) id | from | to | text | datetime
And related user
Tried through union, but duplicates are removed only before union:

the code

public function getSended(){
        return $this->hasMany(Chat::className(),['from'=>'id']);
    }

    public function getReceived(){
        return $this->hasMany(Chat::className(),['to'=>'id']);
    }

    public function getDialogs(){
        return $this->getSended()->select('id,to,to as userid')->groupBy('userid')->union($this->getReceived()->select('id,from,from as userid')->groupBy('userid'))->with('toUser');
    }

a request is received

SELECT COUNT(*) FROM ((SELECT `id`, `to`, `to` AS `userid` FROM `chat` WHERE `from`=1 GROUP BY `userid`) UNION ( SELECT `id`, `from`, `from` AS `userid` FROM `chat` WHERE `to`=1 GROUP BY `userid` )) `c`

Roughly speaking, 2,3,4 are combined with 2,4,5 and get 2,3,4,2,4,5 instead of the desired, 2,3,4,5. groupBy after union does not plow. Jammed me tightly, need a help.
So far I got out of the situation like this:

hellish idiocy

public function getDialogs(){
        $ids = $this->getSended()->select('to as userid')->groupBy('userid')->union($this->getReceived()->select('from as userid')->groupBy('userid'))->asArray()->all();
        return self::find()->andWhere(['id'=>ArrayHelper::getColumn($ids,'userid')])->groupBy('id');
    }

but it's a coma

Answer 1

That is, in fact, you have a table of messages between users and you need to select the last message within each dialog.
I see three solutions here:
Option 1.
If we assume that the last message is a message with a large identifier, then all the last messages in the dialog can be obtained like this

SELECT
  *
FROM
  chat
LEFT JOIN (
  SELECT
    max(id) AS message_id,
    IF (`to` = 1, `from`, `to`) AS `collocutor_id`
  FROM
    chat
  GROUP BY
    `collocutor_id`
) AS m ON chat.id = m.message_id
WHERE
  m.message_id IS NOT NULL

or so

SELECT
  *
FROM
  chat
WHERE
  id IN (
    SELECT
      max(id) AS message_id
    FROM
      chat
    GROUP BY
    IF (`to` = 1, `from`, `to`)
  )

Option 2.
If we assume that the last message is a message with a greater date, then all the latest messages in the dialogue can be obtained like this

SELECT
  chat.*
FROM
  chat
LEFT JOIN (
  SELECT
    max(datetime) AS datetime,

  IF (`to` = 1, `from`, `to`) AS `collocutor_id`
  FROM
    chat
  GROUP BY
    `collocutor_id`
) AS m ON (
  m.`collocutor_id` = chat.`to`
  OR m.`collocutor_id` = chat.`from`
)
AND chat.datetime < m.datetime
WHERE
  m.datetime IS NULL

But here it is worth understanding that if the date and time coincide within the framework of one dialogue, there will be a bug. Although I think the probability of this is extremely small
. Option 3.
Redesign the database, namely, create a separate table containing information about the dialogs and work directly with it. This would avoid unnecessary queries to the database