containedIn query for Firebase - ios

Question

Coming from Parse, I have heavily relied on the containedIn query to collect the right data. In Parse, I might have had an array of objectIds and queried for all objects with those ids. I am looking to achieve the same on Firebase.
I understand that it is important to flatten data, but I don't see how this helps with the problem. Let's say I have a chat room with a list of users inside it. I collect that data and now have an array of usernames. I would like to now navigate to the users in the database and retrieve all that match one element inside this username array. How can I accomplish something like this?
For example, a set of users in a official Firebase example:
{
"users": {
"alovelace": { ... },
"ghopper": { ... },
"eclarke": { ... }
}
}
I would like to perform a query to download the following users:
["alovelace", "eclarke"]
While a general answer would be helpful, an answer in Swift would be best. Thank you.

Answer 1

An example is that they are the two members of a chat room. Or that
the current user is following them.
So a theoretical users node
users
alovelace
followed_by
bill: true
frank: true
in_chat_room: room_42
location: France
ghopper
followed_by
jay: true
in_chat_room: room_27
location: USA
eclarke
followed_by
frank: true
in_chat_room: room_42
location: Canada
and some chat rooms
chat_rooms
room_27
ghopper: true
room_42
lovelace: true
eclarke: true
To get the detailed users nodes of users in chat room 42 (lovelace, eclarke)
let usersRef = self.myRootRef.childByAppendingPath("users")
usersRef.queryOrderedByChild("in_chat_room").queryEqualToValue("room_42")
.observeEventType(.Value, withBlock: { snapshot in
for child in snapshot.children {
let location = child.value["location"] as! String
print(location) //prints France and Canada
}
})
To get the users Frank is following (lovelace, eclarke):
let usersRef = self.myRootRef.childByAppendingPath("users")
usersRef.queryOrderedByChild("followed_by/frank").queryEqualToValue(true)
.observeEventType(.Value, withBlock: { snapshot in
for child in snapshot.children {
let userName = child.key as String
print(userName)
}
})
Note that using user names as node key's is generally a bad idea - they should be stored by their uid.
Also note we didn't really do anything with the chat_rooms node but to maintain the relationship, having nodes refer to each other can be useful for observing changes etc.
Edit:
In response to a comment, here's the structure for each user to show who they are following instead of who is following the user
users
alovelace
following_user
bill: true
frank: true
in_chat_room: room_42
location: France
with this structure, alovelace is following bill and frank.
To get all of the users following frank:
let usersRef = self.myRootRef.childByAppendingPath("users")
usersRef.queryOrderedByChild("following_user/frank").queryEqualToValue(true)
.observeEventType(.Value, withBlock: { snapshot in
for child in snapshot.children {
let userName = child.key as String
print(userName)
}
})