Is there an `exists` where clause I can use with firestore?

Question

Use case: find conversations where I am a participant.

Given a collection of conversations is there a way to use a where exists clause on a nested object's properties? I am using an object with an id as it's key hoping that it would be something I can run a where query against. Should I have put the participants field in an array instead?

 db.collection('conversations')
            .where('participants.${userId}', 'exists', true)
            .get()
            .then(() => {
                // ...
            });

Sample conversation object

{ 
  id: 'foo'
  participants: { 
    userIdA: {
       username: 'bar'
    },
    userIdB: {
       username: 'joe'
    }
  }
}

Answer 1

Assuming that your sample conversation object is a complete example, all you would have to do is set each participating userId equal to true , and then have your query like this:

db.collection('conversations')
  .where(`participants.${userId}`, ==, true)
  .get()
  .then(snapshot => {
    // ...
  });

If you needed to know more about a specific user, such as their name, you could just look it up in a users collection, since you know what their ID is.

db.doc(`users/${userId}`)
  .get()
  .then(snapshot => {
    // ...
  });