FIRDatabaseQuery: how to do an inner join - ios

Question

I'm trying to do an inner join on a FIRDatabaseQuery object.
below is the database structure. I have some posts that are linked to post-comments. I am trying to get all the posts that a specific user added a comment on:
{
"posts" : {
"-KIycKhZU55dmKnHbbxv" : {
"author" : "John Doe",
"body" : "This is a post test",
"title" : "test",
"uid" : "SHaH36BLwgPvwgi9cDmRnsnONFB2"
},
"-KIyg_ks1O5QL4_4dfq_" : {
"author" : "Jane Doe",
"body" : "This is a post test",
"title" : "test2",
"uid" : "x5leSBGArnd10JilD9YDyNBNfZ03"
},...
}
"post-comments" : {
"-KIycKhZU55dmKnHbbxv" : {
"-KIycMyL0Vy1BHVdI4zc" : {
"author" : "toto",
"text" : "test",
"uid" : "SHaH36BLwgPvwgi9cDmRnsnONFB2"
},
"-KIyg_ks1O5QL4_4dfq_" : {
"author" : "toto",
"text" : "test",
"uid" : "SHaH36BLwgPvwgi9cDmRnsnONFB2"
}
},...
}
in SQL this will be translated into a inner join query similar to:
Select * from post inner join post-comments on post-id where post-comments.uid = "user id"
Does anybody know how o get something similar to an inner join in firebase?
Thanks a lot,
Yacine

Answer 1

You may want to consider
"posts" : {
"-KIycKhZU55dmKnHbbxv" : {
"author" : "John Doe",
"body" : "This is a post test",
"title" : "test",
"uid" : "SHaH36BLwgPvwgi9cDmRnsnONFB2"
"commented_by"
"SHaH36BLwgPvwgi9cDmRnsnONFB2": true
Then you can simply query for
posts.child("commented_by/uid").queryEqualToValue(true)
or, change the post-comments around to better match the queries you want to do:
"post-comments" : {
"-KIycMyL0Vy1BHVdI4zc" : {
"author" : "toto",
"post_data":
post_id: "-KIycKhZU55dmKnHbbxv",
post_title: "test"
"text" : "test",
"uid" : "SHaH36BLwgPvwgi9cDmRnsnONFB2"
},
That makes the query a snap as the post-comments node can be queried for the uid which return all of the post_id's of the posts they commented on. It wouldn't return the post itself, however, you may just be looking for the title so you can populate a list. Once the user taps it clicks it you can retrieve the actual data.

Answer 2

You will need to work with nested firebase calls. You can find a javascript example in this question but your swift code should look like the following:
ref.child("posts").observeEventType(.ChildAdded, withBlock: { (snapshot) in
if let postId = snapshot.key as! String {
let commentsRef = ref.child("post-comments")
commentsRef.child(postId).queryOrderedByChild("uid").queryEqualToValue(userId).observeSingleEventOfType(.Value, withBlock: { (snapshot) in
for child in snapshot.children.allObjects as [FDataSnapshot] {
print(child.value)
}
}) { (error) in
print(error.localizedDescription)
}
}
})