Querying in Firebase by child of child - swift

Question

I have a structure of objects in Firebase looking like this:
-KBP27k4iOTT2m873xSE
categories
Geography: true
Oceania: true
correctanswer: "Yaren (de facto)"
languages: "English"
question: "Nauru"
questiontype: "Text"
wronganswer1: "Majuro"
wronganswer2: "Mata-Utu"
wronganswer3: "Suva"
I'm trying to find objects by categories, so for instance I want all objects which has the category set to "Oceania".
I'm using Swift and I can't really seem to grasp the concept of how to query the data.
My query right now looks like this:
ref.queryEqualToValue("", childKey: "categories").queryOrderedByChild("Oceania")
Where ref is the reference to Firebase in that specific path.
However whatever I've tried I keep getting ALL data returned instead of the objects with category Oceania only.
My data is structured like this: baseurl/questions/
As you can see in the object example one question can have multiple categories added, so from what I've understood it's best to have a reference to the categories inside your objects.
I could change my structure to baseurl/questions/oceania/uniqueids/, but then I would get multiple entries covering the same data, but with different uniqueid, because the question would be present under both the categories oceania and geography.
By using the structure baseurl/questions/oceania/ and baseurl/questions/geography I could also just add unique ids under oceania and geography that points to a specific unique id inside baseurl/questions/uniqueids instead, but that would mean I'd have to keep track of a lot of references. Making a relations table so to speak.
I wonder if that's the way to go or? Should I restructure my data? The app isn't in production yet, so it's possible to restructure the data completely with no bigger consequences, other than I'd have to rewrite my code, that pushes data to Firebase.
Let me know, if all of this doesn't make sense and sorry for the wall of text :-)

Answer 1

Adding some additional code to Tim's answer for future reference.
Just use a deep query. The parent object key is not what is queried so it's 'ignored'. It doesn't matter whether it's a key generated by autoId or a dinosaur name - the query is on the child objects and the parent (key) is returned in snapshot.key.
Based on your Firebase structure, this will retrieve each child nodes where Oceania is true, one at a time:
let questionsRef = Firebase(url:"https://baseurl/questions")
questionsRef.queryOrderedByChild("categories/Oceania").queryEqualToValue(true)
.observeEventType(.ChildAdded, withBlock: { snapshot in
print(snapshot)
})
Edit: A question came up about loading all of the values at once (.value) instead of one at at time (.childAdded)
let questionsRef = Firebase(url:"https://baseurl/questions")
questionsRef.queryOrderedByChild("categories/Oceania").queryEqualToValue(true)
.observeSingleEventOfType(.Value, withBlock: { snapshot in
print(snapshot)
})
Results in (my Firebase structure is a little different but you get the idea) uid_1 did not have Oceania = true so it was omitted from the query
results.
Snap (users) {
"uid_0" = {
categories = {
Oceania = 1;
};
email = "dude#thing.com";
"first_name" = Bill;
};
"uid_2" = {
categories = {
Oceania = 1;
};
"first_name" = Peter;
};
}

Answer 2

I think this should work:
ref.queryOrderedByChild("categories/Oceania").queryEqualToValue(true)