Firestore: Query by item in array of document

Firestore has now added an 'in' query as of November 2019. According to the announcement article:

With the in query, you can query a specific field for multiple values (up to 10) in a single query. You do this by passing a list containing all the values you want to search for, and Cloud Firestore will match any document whose field equals one of those values.

Added 'array-contains' query operator for use with .where() to find documents where an array field contains a specific element.

https://firebase.google.com/support/release-notes/js 5.3.0

Update: also available in @google-cloud/firestore: https://github.com/googleapis/nodejs-firestore/releases/tag/v0.16.0

Update 2 https://firebase.googleblog.com/2018/08/better-arrays-in-cloud-firestore.html

Update 3 now available in Admin Node.js SDK v6.0.0 https://github.com/firebase/firebase-admin-node/releases

Here is a bit of expansion on the answer as some seem to be confused about having to make indexes for each key, Firestore already indexes your data for simple queries thus you can do a simple query like

documentReference.where('param','==','value').onSnapshot(...)

but you can not do a compound query unless you index your data for those parameters. So you would need indexes to be able to do something like this:

 documentReference.where('param','==','value').where(..otherparams...).onSnapshot(...)

So as long as you need the photos for an id you can save them as

usersCollection :                        (a collection)
    uidA:                                (a document)
         photoField:                     (a field value that is a map or object)
            fieldID1 : true              (a property of the photoField)
            fieldID2 : true              (a property of the photoField)
            etc ...  

and you can simply query user(s) that have, let's say, fieldID1 in their photoField without needing to form any index and like query below.

firestore.doc('usersCollection/uidA').where('photoField.fieldID1','==',true).onSnapshot(...)

See Henry's answer, as we've no made Array Contains queries available.

Unfortunately not yet, although it's on our roadmap.

In the meantime, you'll need to use a map instead, in the form of:

photos: {
    id1: true
    id2: true
}

Now you can find all users with id1 by filtering by photos.id1 == true.

Read more about querying such sets in the Firebase documentation.