I'm trying to test out Firebase to allow users to post comments using push. I want to display the data I retrieve with the following;
fbl.child('sell').limit(20).on("value", function(fbdata) {
// handle data display here
}
The problem is the data is returned in order of oldest to newest - I want it in reversed order. Can Firebase do this?
Since this answer was written, Firebase has added a feature that allows ordering by any child or by value. So there are now four ways to order data: by key, by value, by priority, or by the value of any named child. See this blog post that introduces the new ordering capabilities.
The basic approaches remain the same though:
1. Add a child property with the inverted timestamp and then order on that.
2. Read the children in ascending order and then invert them on the client.
Firebase supports retrieving child nodes of a collection in two ways:
by name
by priority
What you're getting now is by name, which happens to be chronological. That's no coincidence btw: when you push an item into a collection, the name is generated to ensure the children are ordered in this way. To quote the Firebase documentation for push:
The unique name generated by push() is prefixed with a client-generated timestamp so that the resulting list will be chronologically-sorted.
The Firebase guide on ordered data has this to say on the topic:
How Data is Ordered
By default, children at a Firebase node are sorted lexicographically by name. Using push() can generate child names that naturally sort chronologically, but many applications require their data to be sorted in other ways. Firebase lets developers specify the ordering of items in a list by specifying a custom priority for each item.
The simplest way to get the behavior you want is to also specify an always-decreasing priority when you add the item:
var ref = new Firebase('https://your.firebaseio.com/sell');
var item = ref.push();
item.setWithPriority(yourObject, 0 - Date.now());
Update
You'll also have to retrieve the children differently:
fbl.child('sell').startAt().limitToLast(20).on('child_added', function(fbdata) {
console.log(fbdata.exportVal());
})
In my test using on('child_added' ensures that the last few children added are returned in reverse chronological order. Using on('value' on the other hand, returns them in the order of their name.
Be sure to read the section "Reading ordered data", which explains the usage of the child_* events to retrieve (ordered) children.
A bin to demonstrate this: http://jsbin.com/nonawe/3/watch?js,console
Since firebase 2.0.x you can use limitLast() to achieve that:
fbl.child('sell').orderByValue().limitLast(20).on("value", function(fbdataSnapshot) {
// fbdataSnapshot is returned in the ascending order
// you will still need to order these 20 items in
// in a descending order
}
Here's a link to the announcement: More querying capabilities in Firebase
To augment Frank's answer, it's also possible to grab the most recent records--even if you haven't bothered to order them using priorities--by simply using endAt().limit(x) like this demo:
var fb = new Firebase(URL);
// listen for all changes and update
fb.endAt().limit(100).on('value', update);
// print the output of our array
function update(snap) {
var list = [];
snap.forEach(function(ss) {
var data = ss.val();
data['.priority'] = ss.getPriority();
data['.name'] = ss.name();
list.unshift(data);
});
// print/process the results...
}
Note that this is quite performant even up to perhaps a thousand records (assuming the payloads are small). For more robust usages, Frank's answer is authoritative and much more scalable.
This brute force can also be optimized to work with bigger data or more records by doing things like monitoring child_added/child_removed/child_moved events in lieu of value, and using a debounce to apply DOM updates in bulk instead of individually.
DOM updates, naturally, are a stinker regardless of the approach, once you get into the hundreds of elements, so the debounce approach (or a React.js solution, which is essentially an uber debounce) is a great tool to have.
There is really no way but seems we have the recyclerview we can have this
query=mCommentsReference.orderByChild("date_added");
query.keepSynced(true);
// Initialize Views
mRecyclerView = (RecyclerView) view.findViewById(R.id.recyclerView);
mManager = new LinearLayoutManager(getContext());
// mManager.setReverseLayout(false);
mManager.setReverseLayout(true);
mManager.setStackFromEnd(true);
mRecyclerView.setHasFixedSize(true);
mRecyclerView.setLayoutManager(mManager);
I have a date variable (long) and wanted to keep the newest items on top of the list. So what I did was:
Add a new long field 'dateInverse'
Add a new method called 'getDateInverse', which just returns: Long.MAX_VALUE - date;
Create my query with: .orderByChild("dateInverse")
Presto! :p
You are searching limitTolast(Int x) .This will give you the last "x" higher elements of your database (they are in ascending order) but they are the "x" higher elements
if you got in your database {10,300,150,240,2,24,220}
this method:
myFirebaseRef.orderByChild("highScore").limitToLast(4)
will retrive you : {150,220,240,300}
In Android there is a way to actually reverse the data in an Arraylist of objects through the Adapter. In my case I could not use the LayoutManager to reverse the results in descending order since I was using a horizontal Recyclerview to display the data. Setting the following parameters to the recyclerview messed up my UI experience:
llManager.setReverseLayout(true);
llManager.setStackFromEnd(true);
The only working way I found around this was through the BindViewHolder method of the RecyclerView adapter:
#Override
public void onBindViewHolder(final RecyclerView.ViewHolder holder, int position) {
final SuperPost superPost = superList.get(getItemCount() - position - 1);
}
Hope this answer will help all the devs out there who are struggling with this issue in Firebase.
Firebase: How to display a thread of items in reverse order with a limit for each request and an indicator for a "load more" button.
This will get the last 10 items of the list
FBRef.child("childName")
.limitToLast(loadMoreLimit) // loadMoreLimit = 10 for example
This will get the last 10 items. Grab the id of the last record in the list and save for the load more functionality. Next, convert the collection of objects into and an array and do a list.reverse().
LOAD MORE Functionality: The next call will do two things, it will get the next sequence of list items based on the reference id from the first request and give you an indicator if you need to display the "load more" button.
this.FBRef
.child("childName")
.endAt(null, lastThreadId) // Get this from the previous step
.limitToLast(loadMoreLimit+2)
You will need to strip the first and last item of this object collection. The first item is the reference to get this list. The last item is an indicator for the show more button.
I have a bunch of other logic that will keep everything clean. You will need to add this code only for the load more functionality.
list = snapObjectAsArray; // The list is an array from snapObject
lastItemId = key; // get the first key of the list
if (list.length < loadMoreLimit+1) {
lastItemId = false;
}
if (list.length > loadMoreLimit+1) {
list.pop();
}
if (list.length > loadMoreLimit) {
list.shift();
}
// Return the list.reverse() and lastItemId
// If lastItemId is an ID, it will be used for the next reference and a flag to show the "load more" button.
}
I'm using ReactFire for easy Firebase integration.
Basically, it helps me storing the datas into the component state, as an array. Then, all I have to use is the reverse() function (read more)
Here is how I achieve this :
import React, { Component, PropTypes } from 'react';
import ReactMixin from 'react-mixin';
import ReactFireMixin from 'reactfire';
import Firebase from '../../../utils/firebaseUtils'; // Firebase.initializeApp(config);
#ReactMixin.decorate(ReactFireMixin)
export default class Add extends Component {
constructor(args) {
super(args);
this.state = {
articles: []
};
}
componentWillMount() {
let ref = Firebase.database().ref('articles').orderByChild('insertDate').limitToLast(10);
this.bindAsArray(ref, 'articles'); // bind retrieved data to this.state.articles
}
render() {
return (
<div>
{
this.state.articles.reverse().map(function(article) {
return <div>{article.title}</div>
})
}
</div>
);
}
}
There is a better way. You should order by negative server timestamp. How to get negative server timestamp even offline? There is an hidden field which helps. Related snippet from documentation:
var offsetRef = new Firebase("https://<YOUR-FIREBASE-APP>.firebaseio.com/.info/serverTimeOffset");
offsetRef.on("value", function(snap) {
var offset = snap.val();
var estimatedServerTimeMs = new Date().getTime() + offset;
});
To add to Dave Vávra's answer, I use a negative timestamp as my sort_key like so
Setting
const timestamp = new Date().getTime();
const data = {
name: 'John Doe',
city: 'New York',
sort_key: timestamp * -1 // Gets the negative value of the timestamp
}
Getting
const ref = firebase.database().ref('business-images').child(id);
const query = ref.orderByChild('sort_key');
return $firebaseArray(query); // AngularFire function
This fetches all objects from newest to oldest. You can also $indexOn the sortKey to make it run even faster
I had this problem too, I found a very simple solution to this that doesn't involved manipulating the data in anyway. If you are rending the result to the DOM, in a list of some sort. You can use flexbox and setup a class to reverse the elements in their container.
.reverse {
display: flex;
flex-direction: column-reverse;
}
myarray.reverse(); or this.myitems = items.map(item => item).reverse();
I did this by prepend.
query.orderByChild('sell').limitToLast(4).on("value", function(snapshot){
snapshot.forEach(function (childSnapshot) {
// PREPEND
});
});
Someone has pointed out that there are 2 ways to do this:
Manipulate the data client-side
Make a query that will order the data
The easiest way that I have found to do this is to use option 1, but through a LinkedList. I just append each of the objects to the front of the stack. It is flexible enough to still allow the list to be used in a ListView or RecyclerView. This way even though they come in order oldest to newest, you can still view, or retrieve, newest to oldest.
You can add a column named orderColumn where you save time as
Long refrenceTime = "large future time";
Long currentTime = "currentTime";
Long order = refrenceTime - currentTime;
now save Long order in column named orderColumn and when you retrieve data
as orderBy(orderColumn) you will get what you need.
just use reverse() on the array , suppose if you are storing the values to an array items[] then do a this.items.reverse()
ref.subscribe(snapshots => {
this.loading.dismiss();
this.items = [];
snapshots.forEach(snapshot => {
this.items.push(snapshot);
});
**this.items.reverse();**
},
For me it was limitToLast that worked. I also found out that limitLast is NOT a function:)
const query = messagesRef.orderBy('createdAt', 'asc').limitToLast(25);
The above is what worked for me.
PRINT in reverse order
Let's think outside the box... If your information will be printed directly into user's screen (without any content that needs to be modified in a consecutive order, like a sum or something), simply print from bottom to top.
So, instead of inserting each new block of content to the end of the print space (A += B), add that block to the beginning (A = B+A).
If you'll include the elements as a consecutive ordered list, the DOM can put the numbers for you if you insert each element as a List Item (<li>) inside an Ordered Lists (<ol>).
This way you save space from your database, avoiding unnecesary reversed data.
Related
I have a list of vehicle makes, while the list is so large and I want to store it in room database in android. The app may crash while performing this operation. I want to loop through the list and take some chunks of items and to store to database. For example for each loop I take 20 items until the list is empty. How do I achieve this in Kotlin or any other suggestion that can work efficiently.
You don't say if you care in what order you want to remove the list items. If you don't care, then the following code shows how you can achieve it. Notice that the original list must be mutable for the remove() operation to work
val CHUNK_SIZE = 20
val vehicles = (0..177).map { "Car ${it}" }.toMutableList()
val carListIterator = vehicles.iterator()
val removalChunk = mutableListOf<String>()
while(carListIterator.hasNext()) {
removalChunk.add(carListIterator.next())
carListIterator.remove() // this removes the element that was just returned
if(removalChunk.size >= CHUNK_SIZE || !carListIterator.hasNext()) {
//store chunk elsewhere
println("Storing: ${removalChunk.joinToString()}")
removalChunk.clear()
}
}
Advertissement: I'm using CouchbaseLite Android 1.4.0 and I'm not ready to switch to 2.0.
To get the context of my issue:
I have 2 doc types: PATIENT & VISIT
A patient is linked to a visit by a field visitId
A visit has a startDateTime and a endDateTime
A visit is considered as 'open' if the endDateTime is null or missing
A visit can be of different types through the field visitType
A visit can be inactive through the field inactive
I want to:
Get all patients (id + name) of all open and active visits of a certain type
The result is paginated using skip and limit
Get the result sorted by name
I managed to get all the correct patients and to paginate the result but the issue is with the sorting.
I can't make a 'post sorting' (getting the result in a random order and then use self-made method to sort them) because of the pagination.
So here is how it works actually without the sorting:
THE MAP/REDUCE
if (isVisit()) {
Object inactive = document.get(FIELDS.INACTIVE);
if(inactive == null)
inactive = false;
Document patientDoc = database.getExistingDocument(FIELDS.PATIENT_ID);
if(patientDoc != null && patientDoc.getProperties() != null) {
Object[] keys = {
document.get(CouchbaseVisitManager.FIELDS.STOP_DATE_TIME),
inactive,
document.get(CouchbaseVisitManager.FIELDS.VISIT_TYPE)
};
Object[] values = {
document.get(FIELDS.PATIENT_ID),
patientDoc.getProperties().get(FIELDS.FAMILY_NAME),
patientDoc.getProperties().get(FIELDS.FIRST_NAME)
}
emitter.emit(keys, values);
}
}
THE QUERY
Query query = getQuery(Views.PATIENTS_CURRENTLY_IN_VISIT); //Helper method to create a query
query.setKeys(new ArrayList<Object>(){{
add(new ArrayList<Object>(){{
add(null);
add(false);
add(visitType);
}});
}});
query.setSkip(skip);
query.setLimit(limit);
And this is working well to get my patients but not for sorting them.
I tried to add patient names inside the view like this:
Object[] keys = {
patientDoc.getProperties().get(FIELDS.FAMILY_NAME),
patientDoc.getProperties().get(FIELDS.FIRST_NAME),
document.get(FIELDS.STOP_DATE_TIME),
inactive,
document.get(FIELDS.VISIT_TYPE)
};
And update my query.keys like this:
query.setStartKey(new ArrayList<Object>(){{
add(null);
add(null);
add(null);
add(false);
add(visitType);
}});
query.setEndKey(new ArrayList<Object>(){{
add(new HashMap<>());
add(new HashMap<>());
add(null);
add(false);
add(visitType);
}});
But this returns no patients (result.size = 0)
So... I don't know how to achieve my goal.
I tought of a way to sort by values but it doesn't seem to exist yet (should wait for 2.0 I think). But is their any workaround to achieve this kind of behavior?
Thank's for reading.
View results are always sorted by key. So you should add the patient name to the key and then you'll get your sorting.
Be careful to put the name into the correct place of the keys array since the sorting is lexicographical, that is if the name is going to be the last component then you'll get all open inactive sorted by name, then all open active sorted by name e.t.c.
In case you need to put your name key in front of the other keys but still be able to filter your results, then you will need to properly construct the start key and the end key so that the full range of every possible patient names would be accepted while the later keys would be filtered.
You will have to provide some values which will definitely be less than or greater than any possible patient name.
Following the key collation defined for Couchbase views, a null will be always less than any string, and an empty map {} can be used to match past the last string. You are basically doing it right in your last sample.
Don't forget to switch the start and end keys logic if you would like to query the view in reverse mode
Another thing to take care of is always incrementing the version of the view after changing its logic or key set, otherwise you will continue getting old results. Incrementing the version will force a refresh of the view, executing the updated logic in the view function over all your data.
I need to have one problem sorted out. I want a cloud function, where I need to loop through every Posts that has uniqueID and I want to sort those uniqueID's according to the value of TotalReactions. To me more vivid, suppose, the first post has totalReactions = 5, and second one has totalReactions = 6, then I need to have another RealTimeDatabase "PostArranged" which sorts them in descending order, so second post which has Total Reaction would be in first and then follows the one which has lesser TotalReaction...
Here is how my firebase realtime database looks like:
Posts
-L29TD-nsUYRu3wYcCQl
Caption: "First Screenshot"
CurrentUserReaction: "notreacted"
Image: "https://firebasestorage.googleapis.com/v0/b/nep..."
ReactingUser
<user-id>:"1"
<user-id>:"2"
Time: "13:40:54"
TotalReactions: "2"
Unique: "-L29TD-nsUYRu3wYcCQl"
UserPhoto: "https://firebasestorage.googleapis.com/v0/b/nep..."
Username: "Me"
Any help would be appreciated ..
This is not a good way to do what you want to do. In this method the database has to be sorted to another "ArrangedDatabase" every time a reaction is added which will happen in a high frequency. So it will be very expensive to do this with Cloud Functions.
Instead use orderByChild(), orderByKey(), orderByValue() to sort data when you're querying data.
Sorting data - Firebase documentation this show it quite well.
For anyone who want to do this in your RecyclerViewAdapter provided by Firebase, it is easily done... When you pass in those 4 parameters. Do not pass the database Reference, instead, make a new query, and make sure to sort that thing either by OrderByChild(), OrderByValue() or anything you want to order by... i.e.
In my case, I will do something like this:
q = mDatabaseReference.orderByChild("TotalReactions");
and pass q as parameter inside FirebaseAdapter, like this:
FirebaseRecyclerAdapter<Posts,PostViewHolder> firebaseRecyclerAdapter = new FirebaseRecyclerAdapter<Posts, PostViewHolder>(
Posts.class, R.layout.each_post_layout , PostViewHolder.class, q
) {
.
}
I have followed the Android Quickstart provided by google Google Sheets API Android Quickstart and was able to retrieve data from the google spreadsheet but I am not able to understand how to write and update single or multiple data.
I read this code from StackOverflow, I think It's good but I can't understand how to set (valueRange) object here
this.mService.spreadsheets().values().update(spreadsheetId, range, valueRange)
.setValueInputOption("RAW")
.execute();
In case you still need the answer, or for anyone else who does:
I faced the same issue too, and from the docs, I was able to work this out.
Here's a method in which i write the data to the sheet
private void writeDataToApi() throws IOException {
String spreadsheetId = "the_spreadsheet_id_like_the_google_example";
String range = "YourSheetName!A1:B1"; //Read the docs on how these ranges work.
//Currently, this is the range of a single row which would return
//as [[objA, objB]] if major dimension is set as ROW.(default).
// would be [[objA],[objB]] if its set to COLUMN. Read the doc for more info.
//for the values that you want to input, create a list of object lists
List<List<Object>> values = new ArrayList<>();
//Where each value represents the list of objects that is to be written to a range
//I simply want to edit a single row, so I use a single list of objects
List<Object> data1 = new ArrayList<>();
data1.add("objA");
data1.add("objB");
//There are obviously more dynamic ways to do these, but you get the picture
values.add(data1);
//Create the valuerange object and set its fields
ValueRange valueRange = new ValueRange();
valueRange.setMajorDimension("ROWS");
valueRange.setRange(range);
valueRange.setValues(values);
//then gloriously execute this copy-pasted code ;)
this.mService.spreadsheets().values()
.update(spreadsheetId, range, valueRange)
.setValueInputOption("RAW")
.execute();
//Try calling this method before executing the readDataFromApi method,
//and you'll see the immediate change
}
I hope this helped.
And for more info, pls see the docs
EDIT: Also remember to change scope from SheetsScopes.SPREADSHEETS_READONLY to SheetsScopes.SPREADSHEETS
I have a simple firebase database: /rides is a list of simple objects like this
{
car: "Toyota"
minutes: 15
}
and I need to display sum of minutes of all the rides. The obvious solution is to load all the rides and calculate the sum. But if I have several hundreds of rides this is very slow, up to several seconds.
So it seems I have to maintain a separate field /totalMinutesin the database for this. But thus I will have to manually update /totalMinutes every time I add/remove/change a ride. Anyway this is not a big deal of work.
But what if I need to calculate total minutes only for a subset of rides? For instance only for "Toyota" cars or "Ford" cars? Manual maintaining /totalMinutesFord, /totalMinutesToyota now doesn't seem so easy.
So what is the correct way to maintain such dynamic values in firebase?
Firebase has no way to get automatically calculate values based on the data in your database.
So your two options are:
calculate the value whenever you update the data
retrieve all the data and calculate the value on the client
You already (wisely) decided that retrieving all data is not a good idea. Your users will be grateful for that.
So that leaves calculating the derived values whenever you update the data of a ride. I'm not sure why doing that for multiple values would be more difficult than doing it for a single value. It may be more code, but it's pretty much the same code:
var ride = { car: "Toyota", minutes: 15 };
ref = new Firebase('https://yours.firebaseio.com/');
ref.child('rides').push(ride);
ref.child('totalMinutes').transaction(function(current_value) {
return (current_value || 0) + ride.minutes;
});
ref.child('totalMinutes'+ride.car).transaction(function(current_value) {
return (current_value || 0) + ride.minutes;
})