Android Firebase查询范围（上，中，下）

Question

I just start using Firebase in Android and I wrote query like this.

messageSearchQuery = FirebaseDatabase.getInstance().getReference().child("Message")
    .child(conversationID).limitToLast(INITIAL_MESSAGE_COUNT)
    .orderByChild("timestamp").startAt(participant.joiningDate).endAt(endAt);

I can only give limitToFirst , limitToLast , startAt , endAt . (I think there is no center)

I need to give timestamp (eg 10 am sharp) and i wanna query 10 message before 10 am and 10 message after 10 am. (but i don't know when it start, when it end).

Edit

What I want to do is something like this.

messageSearchQuery = FirebaseDatabase.getInstance().getReference().child("Message").
                    child(conversationID).
                    orderByChild("timestamp").equalTo(endAt).limitToFirst(INITIAL_MESSAGE_COUNT).limitToLast(INITIAL_MESSAGE_COUNT);

But if I do, it crash like this.

java.lang.IllegalArgumentException: Can't call limitToLast on query with previously set limit!

How shall I do?

My data in firebase is something like this.

Answer 1

You can select a range by combining startAt and endAt , both of which operate on the child you order on. For example if you want messages between 10am and 10pm on a certain date, you'd do:

long timestampOf10am = ...;
long timestampOf10pm = ...;
messageSearchQuery = FirebaseDatabase.getInstance().getReference().child("Message")
  .child(conversationID)
  .orderByChild("timestamp").startAt(timestampOf10am).endAt(timestampOf10pm)
  .limitToLast(INITIAL_MESSAGE_COUNT);

Update : If you need 10 items before 10am and 10 items after 10am, you'll need to fire two queries and merge the results client-side.

long timestampOf10am = ...;
beforeMessageSearchQuery = FirebaseDatabase.getInstance().getReference().child("Message")
  .child(conversationID)
  .orderByChild("timestamp").startAt(timestampOf10am)
  .limitToFirst(INITIAL_MESSAGE_COUNT);
afterMessageSearchQuery = FirebaseDatabase.getInstance().getReference().child("Message")
  .child(conversationID)
  .orderByChild("timestamp").endAt(timestampOf10am)
  .limitToLast(INITIAL_MESSAGE_COUNT);