Query posts for tags

tagsthenpostsquery.swift

// Assumption: you have nice business objects for Posts and Tags that you build from Firebase snapshots
func posts(forTag tagKey: String, completionBlock: (([Post]) -> ()) {
  let userId = FIRAuth.auth().currentUser?.uid
  let ref = FIRDatabase.database().reference()
  
  var posts = [Post]() //I can put all the posts here as I retrieve them
  
  ref.child("users").child(userId!).child("tags").child(tagKey).observeSingleEvent(of: .value, with: {(tagSnapshot) in
      let tag = Tag(fromSnapshot: tagSnapshot)
      
      for postKey in tags.postKeys {
        ref.child("users").child(userId!).child("posts").child(postKey).observeSingleEvent(of: .value, with: {(postSnapshot)
           //This is async. CRAP HOW DO I GET IT TO WAIT FOR EACH OF THESE TO BE DONE BEFORE CALLING THE COMPLETION BLOCK?!
                                                                                                      
        })                                                                                           
      }
       
      completionBlock(posts) //This will be empty because it will run before any of the post queries run. If I stick it in the inner completionBlock, the consumer will only get one Post.                                                                                             
  })
}