How to find a record through an association? - ruby-on-rails

Question

There are four tables - chat_rooms, chat_messages, chat_rooms_and_users and users:
chat_rooms - The rooms have messages and users.
chat_rooms_and_users - Through this table users are connected to rooms.
Each room can have two users.
How to find a room knowing two users?
I tried like this:
room = joins(:chat_rooms_and_users)
.find_by(
type: ChatRoom.types[:private],
chat_rooms_and_users: {
user: [user_a, user_b]
}
)
SELECT "chat_rooms".* FROM "chat_rooms" INNER JOIN "chat_rooms_and_users" ON "chat_rooms_and_users"."room_id" = "chat_rooms"."id" WHERE "chat_rooms"."type" = $1 AND "chat_rooms_and_users"."user_id" IN ($2, $3) LIMIT $4 [["type", 0], ["user_id", 497], ["user_id", 494], ["LIMIT", 1]]
This bothers me in the SQL code:
"chat_rooms_and_users"."room_id" = "chat_rooms"."id"
If there are no rooms, then the first room is created normally. But then there is always only the room whose ID is first than the rest.

Answer 1

Your issue is "chat_rooms_and_users"."user_id" IN ($2, $3) will return all "private" rooms where either user is present.
Instead you want to find a chatroom where both are present.
I would recommend making a scope for this
#assumed
class User < ApplicationRecord
has_many :chat_rooms_and_users
end
class ChatRoom < ApplicationRecord
scope :private_by_users, ->(user_a,user_b) {
where(type: ChatRoom.types[:private])
.where(id: user_a.chat_rooms_and_users.select(:chat_room_id))
.where(id: user_b.chat_rooms_and_users.select(:chat_room_id))
}
end
#Then
ChatRoom.private_by_users(user_a,user_b)
This will return a collection of "private" rooms where user_a and user_b are both participants. The SQL will look akin to:
SELECT "chat_rooms".*
FROM "chat_rooms"
WHERE "chat_rooms"."type" = 0 AND
"chat_rooms"."id" IN (
SELECT
"chat_rooms_and_users"."chat_room_id"
FROM
"chat_rooms_and_users"
WHERE
"chat_rooms_and_users"."user_id" = user_a_id
) AND "chat_rooms"."id" IN (
SELECT
"chat_rooms_and_users"."chat_room_id"
FROM
"chat_rooms_and_users"
WHERE
"chat_rooms_and_users"."user_id" = user_b_id
)
If you can guarantee there will only ever be 1 or 0 rooms, of this type, with both participants then you can add first to the end of this chain.