|chat_id|user_id|
-----------------
|   1   |   3   |
|   1   |   4   |
|   2   |   3   |
|   2   |   6   |

|chat_id|sender_id|      body    |     created_at     |
------------------------------------------------------|
|   1   |   4     |"initial 1   "|2019-06-30 10:31:16 |
|   1   |   3     |"answer day 1"|2019-06-30 10:52:56 |
|   2   |   6     |"initial ans" |2019-06-30 10:54:33 |
|   2   |   3     |"answer"      |2019-06-30 10:56:30 |
|   1   |   4     |"initial 2"   |2019-07-01 09:30:02 |
|   1   |   3     |"answer day 2"|2019-07-01 09:45:02 |