Using Postgres I have a schema that has `conversations` and `conversationUsers`. Each `conversation` has many `conversationUsers`. I want to be able to find the conversation that has the exactly specified number of `conversationUsers`. In other words, provided an array of `userIds` (say, `[1, 4, 6]`) I want to be able to find the conversation that contains only those users, and no more. 

So far I've tried this:

```
SELECT c."conversationId"
FROM "conversationUsers" c
WHERE c."userId" IN (1, 4)
GROUP BY c."conversationId"
HAVING COUNT(c."userId") = 2;

```

Unfortunately, this also seems to return conversations which include these 2 users among others. (For example, it returns a result if the conversation also includes `"userId"` 5).