How to query for more than one item in an array in Firestore? ("array-contains")

Question

I am building a 1-to-1 chat application with Firestore and Javascript (vue.js).

I have a conversation and a message document defined as follows:

.collection("coversation").doc()

conversation: {
  participantsArray: ["id1", "id2"],
  participants: {
    id1: {
      /* details like avatar & name */
    },
    id2: {
      /* details like avatar & name */
    }
  }
}

.collection("coversation").doc("foo").collection("messages").doc()

message: {
  message: "foo"
}

This is great, so I can now query for all conversations of a specific user like so:

db.collection('conversations').where("participantsArray", "array-contains", userId)

The issue I am having now is the following:

I want to query for a specific conversation, like so:

db.collection('conversations')
  .where("participantsArray", "array-contains", userId)
  .where("participantsArray", "array-contains", chatpartnerId)

But this does not work since double "array-contains" is not allowed and throws an error message.

What works for me for now is to make two calls like so:

db.collection('conversations')
  .where(`participantsArray`, '==', [userId, chatpartnerId])

db.collection('conversations')
  .where(`participantsArray`, '==', [chatpartnerId, userId])

and then check if any of these queries returns a conversation. If so, I use that conversation to add a message, otherwise I create a new conversation.

It works, but it's a bunch of code and quite inefficient.

So my question would be:

1. Is there a better way to do this?
2. How to do "array-contains" to check for multiple items?
3. I have both a participantsArray and a participants field because I did not figure out how to do "array-contains" on an array of objects. Is there a way or should I keep both fields?

PS: Before, I used to query on the objects in the participants object like so:

db.collection("conversations").where(`participants.${userId}.id`, "==", userId)

This was the proposed way to query before "array-contains" existed - but when combined with orderBy I would have to create an index for each user, which is not really an option.

Answer 1

I found a way around how to query for a single conversation.

Since I have both (1) the participants with the details in an "participants" object and (2) the participant ID's in "participantsArray", I can query for a single conversation as follows:

db.collection('conversations')
      .where(`participants.${userId}.id`, "==", userId)
      .where(`participants.${chatPartnerId}.id`, "==", chatPartnerId)

So I am happy for now. However, the three questions remain, so if someone can answer them properly their answer would be very valuable :)