I have this database:
The candidates collection contains several categories such as president and secretary. It also has totalVotes for each candidate.
I would like to query:
Results by category eg President or Secretary
The candidate with highest 'totalVotes' per category, i.e, the President with the highest votes, and the secretary with highest votes.
Here is my code that is not working:
DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference candidatesRef = rootRef.child("candidates");
Query specific = candidatesRef.orderByChild("category").equalTo("President");
specific.orderByChild("totalVotes").limitToFirst(1);
specific.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot){
for (DataSnapshot childSnapshot: dataSnapshot.getChildren()) {
String firstname = childSnapshot.child("firstname").getValue(String.class);
String lastname = childSnapshot.child("lastname").getValue(String.class);
Long votes = childSnapshot.child("totalVotes").getValue(Long.class);
pres.setText(firstname + " " + lastname+" - "+votes+" Votes");
}
}
#Override
public void onCancelled(DatabaseError databaseError) {
throw databaseError.toException(); // don't swallow errors
}
});
How can I achieve this? Thanks
Unfortunately, there is no OR clause in Firebase. So you cannot filter the candidates on the category property based on multiple values.
There is a solution that might work but only if those values are in a range. In your case, you can get all candidates matching President or Secretary with:
ref.child("candidates")
.orderByChild("category")
.startAt("President")
.endAt("Secretary")
The issue here is that it will also match candidates for other categories whose category is between President and Secretary if it exists. Besides that, you can not limit the query to 1 or 2 because you'll never know which candidate will have the highest number of votes. It might be a president, as well as a secretary.
So, there is no way in the Firebase Realtime Database to pass multiple values into a query. You need to perform separate queries for that and merge the result on the client.
If you consider at some point in time to try using Cloud Firestore, you can find there Query's whereIn(String field, List values) method that can help you solve this issue.
Related
I want to retrieve Firebase data within a range of say 1 week. I can query and get data for a day like today, but how about for a range of say 1 week? This is the code that am currenly using for retrieving data for a given day
String mDate = DateFormat.getDateInstance().format(new Date());
DatabaseReference reference = FirebaseDatabase.getInstance().getReference("expenses").child(onlineUserId);
Query query = reference.orderByChild("date").equalTo(mDate);
query.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
myDataList.clear();
for (DataSnapshot snapshot :dataSnapshot.getChildren()){
Data data = snapshot.getValue(Data.class);
myDataList.add(data);
}
todayItemsAdapter.notifyDataSetChanged();
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
});
I cannot find a way of retrieving data for a given range. Someone please help.
The Realtime Database is not designed for complex SQL-like queries.
Not sure if your question refers to grouping results by week. If all you need is a set of results that start and on a certain date and end on another date, you can use startAt(...) and endAt(...) like described in this answer or the spec.
If you need anything more complex than that, you need to
either take all results and filter them in your front end/app code
or use a Cloud Function to do the filtering on the server and
passing the results to the front end.
(Not recommended) You can record the week number (i.e. 45_2020) as a separate field in the document, and filter by that. It's messy and you would have to trust that the front end enters correct info in the field.
I am making an Android app in Android Studio and I want to get specific data from the Firebase. I am trying to search through the Firebase Realtime Database to get all of the Users that have a usercode of 1234 and then getting their name. This is how I have it set up in Firebase:
Firebase Setup image link
Since I am searching for the data where the usercode = 1234 it will return (in the datasnapshot) all of the information about Joe and Lily (since they were the two users that had a usercode of 1234), but it shouldn't return information about Kevin. Here is how I did that:
Query getUsers = ref.orderByChild("usercode").equalTo(1234);
getUsers.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(#NonNull DataSnapshot dataSnapshot) {
if(dataSnapshot.exists()){
//User with usercode = 1234 exists
} else {
//User with that Usercode doesn't exist
}
}
#Override
public void onCancelled(#NonNull DatabaseError databaseError) {
}
});
}
The datasnapshot, if converted to a string would have all of Lily and Joe's information I believe.
What I want to do is get the first user's name (Which would be Joe in this case) from that datasnaphot. My code will then use it through a few tests before going to the second user's name(Which would be Lily) and repeating the tests. Also, if there were 100 Users then I would need to repeat it for every one of those 100 that had a user code of 1234, so I probably need something repeatable.
So somehow I need to find the first value of the child's name from all of the Users that have a usercode of 1234 and then repeat until I get all of the names and run the test on all of them.
I am new to Stackoverflow, so please tell me if I am missing any information that would be helpful in answering the question.
---- EDIT ----
I have now figured out how to do this with Firebase Firestore. It has simpler queries and allows you to do more with the results. If anyone else has the same issue that I stated above, Firebase Firestore is something you might want to try out.
When using the following line of code:
Query getUsers = ref.orderByChild("usercode").equalTo("1234");
You are searching through the Users node, all users that have the usercode property set to "1234". This query will always return no results because you are searching for 1234 as a String and not as a number, as it is stored in the database. The correct query should be:
Query getUsers = ref.orderByChild("usercode").equalTo(1234); //No double quotes
I want to find max value from only last 50 nodes. I used to orderByChild and limitToLast methods but it finds max from all nodes, not only from last 50.
Here my db:
And my code:
database = FirebaseDatabase.getInstance();
reference = database.getReference("sensor");
Query query = reference.orderByChild("hum").limitToLast(50);
query.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for(DataSnapshot myDataSnapshot : dataSnapshot.getChildren())
{
humTemp = myDataSnapshot.child("hum").getValue().toString();
humMax.setText(humTemp);
}
}
public void onCancelled(DatabaseError firebaseError) {
}
});
So, as you see i try to find max value of hum variable from last 50 nodes.
P.S. New nodes are constantly appearing in the database, so the maximum value should change for every 50 values in realtime
If you want the maximum of the 50 most recent nodes, you're trying to order by two properties:
humidity
the time of the node (or its key)
Firebase Database queries can only order/filter on a single property.
In many cases it is possible to combine the values you want to filter on into a single (synthetic) property. For an example of this and other approaches, see my answer here: http://stackoverflow.com/questions/26700924/query-based-on-multiple-where-clauses-in-firebase. But I don't think that's possible for you.
The best I can think of is to just listen for the last 50 nodes, and then re-ordering them on humidity in the client.
I want to use Firebase real-time database in my app project. I want to check if the word which is filled by user exists in database.
Here is my database ss:
Here is my code:
private Button button;
private FirebaseDatabase database;
private DatabaseReference mRef;
private TextView tvResult;
private EditText et1;
et1=(EditText) findViewById(R.id.et1);
button=(Button) findViewById(R.id.btn);
tvResult=(TextView) findViewById(R.id.tvResult);
database=FirebaseDatabase.getInstance();
mRef=database.getReference();
button.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
city=et1.getText().toString();
Query query = mRef.child("app").orderByChild("cities").equalTo(city);
query.addValueEventListener(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
if (dataSnapshot.exists()) {
// dataSnapshot is the "issue" node with all children with id 0
String get= dataSnapshot.getValue().toString();
tvResult.setText(get);
}
}
#Override
public void onCancelled(DatabaseError databaseError) {}
});
}
});
As I said, user fills edittext and after clicking a button, I want to show written city in textview if there is in database.
What you have in your cities is a set of values: a collection of values, where each value is unique, and the order doesn't matter. So it's a single property with multiple values. And while NoSQL databases are all about denormalizing the data, this is actually one of the cases where I'd recommend that you normalize it a bit.
In the Realtime Database you'll typically model a mathematical set as this JSON:
cities: {
london: true,
paris: true,
milan: true,
ny: true,
istanbul: true
}
This data structure has the advantage that:
Each city is by definition unique, since keys must be unique in their location. I.e. there's no way to have london in here twice.
Each city is now stored as a separate property, which means you can query for it separately.
For example, if you have a list of say trips where each trip can have a cities node, and you model it as I've shown, you could query for all cities that include New York with:
firebase.database().ref("cities").orderByChild("cities/ny").equalTo(true)
This will technically work, but will perform pretty badly, because of the way Firebase Realtime Database indexes work. For more on that, and an explanation of how to model this to allow the use-case, have a look at my answer here: Firebase query if child of child contains a value.
I definitely recommend that you check Cloud Firestore out. This use-case is precisely one where Firestore recently released some big improvements. In Firestore you can now store your data set in an array:
["london", "paris", "milan", "ny", "istanbul"]
So say that again you have a collection of trips, and for each trip you have a field cities whose value is the array above. You can now query for trips that include New York with:
firebase.firestore().collection("trips").where("cities", "array-contains", "ny")
And unlike with the Realtime Database, on Firestore this query will scale very well. For more on this see the blog post Better Arrays in Cloud Firestore!.
Unfortunately, there is no query in Firebase real-time database that looks something like this:
mRef.child("app").orderByChild("cities").contains(city)
But for small datasets, there is a workaround that can help you solve this, which is by querying the database to get all the value for your cities property and use contains() method like this:
String str1 = dataSnapshot.child("cities").getValue(String.class);
String str2 = "london";
boolean b = str1.toLowerCase().contains(str2.toLowerCase());
If you'll try to print the value of b, you'll see that is true. But the above examples works well enough only for very small datasets, it doesn't work for large datasets and this is because downloading an entire object to search for fields client-side isn't practical. In this case, I recommend use a third-party search service like Algolia.
I'm looking for something like this:
final DatabaseReference ref = FirebaseDatabase.getInstance().getReferenceFromUrl("https://myproj.firebaseio.com/groceries");
ref.limitToFirst(1).removeValue();
Unfortunately, ref.limitToFirst(1).removeValue(); doesn't exist, only ref.removeValue() does. I want to remove only the first value! How can I do that? I'm sure it's easy & straightforward.
You can only all removeValue() on a DatabaseReference. Calling limitToFirst(1) returns a Query, which does not implement removeValue().
Calling getRef() on that query also won't work, since Query.getRef() returns the location where you executed the query. In other words limitToFirst(1).getRef().removeValue() will remove the entire location, not just the first child.
To delete a specific child node, you will first need to get a DatabaseReference to that child node. And this requires that you execute the query, because only then will you know what child nodes match the query:
DatabaseReference ref = FirebaseDatabase.getInstance().getReferenceFromUrl("https://myproj.firebaseio.com/groceries");
Query query = ref.limitToFirst(1);
query.addListenerForSingleValueEvent(new ValueEventListener() {
#Override
public void onDataChange(DataSnapshot dataSnapshot) {
for (DataSnapshot childSnapshot: dataSnapshot.getChildren()) {
childSnapshot.getRef().removeValue();
}
}
The loop over getChildren() is needed since a query can potentially match multiple child nodes. While your query will match at most one child node, in general there can be any number of results, so the code handles that.