I am using the onUpdate event handler in Firebase Cloud Functions for the Realtime Database. Works great and I get the before / after snapshot values from the change object, as depicted here :
Before change : { '-M0ONRMFJxvClvoFnHP_': true, '-MNxnG-xnFrYOoU_H0U7': false }
After change : { '-M0ONRMFJxvClvoFnHP_': true }
I am curious if there's an "easy" way to get the diff or patch equivalent for this update operation ? Do I have to dive through the Javascript rabbit-hole and dig up a library (any suggestions ?) or is there a built-in feature inside the change object ?
Note : there's a reference to a fieldMask but I am not really seeing any ways to set this up (doesn't seem documented).
Thanks !
Well, I didn't find anything too exciting, so I just wrote the following that fits my specific purpose :
let beforeKeys = Object.keys(change.before.val() || {});
let afterKeys = Object.keys(change.after.val() || {});
let deletions = [];
let insertions = [];
deletions = beforeKeys.filter(item => !afterKeys.includes(item));
insertions = afterKeys.filter(item => !beforeKeys.includes(item));
and then I mapped the elements of each array to an async call for removal and addition to specific references in my realtime DB.
There's probably a more efficient way to achieve this, but that should work for now.
Thanks for coming to my TED Talk. 🙄
I've been struggling with this problem for a while now, can't really find any direct answers anywhere to get around it. I really appreciate any help and more practical code to understand this.
I'm querying documents from a collection in Firestore. Then I'm putting them inside a custom List:
QuerySnapshot snapshot = await cardRef.limit(10).getDocuments();
List<CustomCard> cards =
snapshot.documents.map((doc) => CustomCard.fromDocument(doc)).toList();
Then I display the list like so:
//if cards != null
return Stack(children: cards);
However, I need to block documents from being added to 'cards' IF they contain a key inside a map of 'keys'.
I've tried to do something like this unsuccessfully:
if(snapshot.documents.contains('keys.$currentUserid'))
//dont add to list
But someone said on my previous post here, that I should hide the card on the client side. Is that the best approach since that card would still be added to the list? How would I do that?
in sum: 1) get all documents from a collection, 2) check each for specific key, 3) if they don't have key, add to list.
You're using snapshot.documents, which is a List<DocumentSnapshot> object. So what you're looking for is a way to derive another list from snapshot.documents with some of the documents removed.
My initial search for that flutter filter list, which shows that List.where is what you should be using for that. A simple example on this site: Flutter: Filter list as per some condition
So that'd lead to something like:
QuerySnapshot snapshot = await cardRef.limit(10).getDocuments();
List<DocumentSnapshot> filtered = snapshot.documents.where((doc) => ... /* your filter goes here */ )
List<CustomCard> cards = filtered.map((doc) => CustomCard.fromDocument(doc)).toList();
store.select() emits previous store state.
Is it possible to subscribe to changes from "this point forward" without getting the previous store value?
If you are not interested in the first emitted value, you should be able to use the skip operator:
store.select(...).skip(1)...
skip operators need piping now, you can use skip like this:
store.pipe(select(...), skip(1));
In terms of the 'hacky' part, it is a standard practice in ngrx to set an initial state with properties set to null. and that value gets emitted initially. so the first value you get will be null in these cases.
Alternatively you could also consider skipwhile(https://www.learnrxjs.io/learn-rxjs/operators/filtering/skipwhile) and use it like this:
store.pipe(select(...), skipWhile(val => val === undefined));
where undefined is the initial value of the property you are interested in. Rather than setting the initial value of the property to undefined, you could use null as the initial value as well, and change the above skipwhile() accordingly.
Just sharing my thoughts (and solution) after reading #Niz's answer.
This is a perfect, practical example of how to utilize the difference between null and undefined. When you initialize your state with null, you're basically saying:
I don't care about differentiating the nullable future state from the
initial one. I don't care if the user is null because he has signed
out or because he just didn't sign in
However, in some cases this could be insufficient. Think about a case when you need an asynchronous call (implemented in effects) in order to know if you have an active user session. Based on the selection result, you should determine whether to show a login modal or redirect to a content page. With initial user state set to null, you'd pop up that modal and then immediately hide it when that asynchronous call returns a session value.
With initial state set to undefined you can make that differentiation, saying:
Initially, I know nothing about my state, then it's undefined. When I know it should be empty, then I'll set it to null.
Therefor, as a practical solution, I set everything on the app's initialState to undefined. In the example above, I need to know if the login modal should be displayed after the asynchronous call resolves. skipWhile(val => val === undefined) will do the job for sure, but repeating it over and over again feels a little tedious. Plus, it's not really descriptive to our use case. I created a rxjs-custom-operators.ts with a shortened implementation:
import { Observable } from "rxjs";
import { skipWhile } from "rxjs/operators";
export const skipInitial = () => {
return <T>(source: Observable <T>): Observable<T> => {
return source.pipe(skipWhile(value => value === undefined));
};
};
Usage:
navigateOnLoad(): void {
this.store.pipe(select(selectAuthUser), skipInitial()).subscribe((authUser: CognitoUser) => {
// Navigate to login if !authUser, else navigate to content...
});
}
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.
I have an object of message streams that looks like this:
ractive.data.messages:
{
stream_id1: {
some_stream_metadata: "foo",
stream: [
{id: "someid1", message: "message1"},
{id: "someid2", message: "message2"}
]
},
stream_id2: {
some_stream_metadata: "bar",
stream: [
{id: "someid3", message: "message3"},
{id: "someid4", message: "message4"}
]
}
}
main_template:
{{#messages[ current_stream_id ]}}
{{>render_message_stream}}
{{/messages[ current_stream_id ]}}
render_message_stream:
{{#stream}}
<div class="stream">
...someotherstuff...
{{>render_message}}
</div>
{{/stream}}
render_message:
<div class="message">
...someotherstuff...
{{message}}
</div>
I change "current_stream_id" to change the rendered stream of messages.
On updates, i change the contents of the message streams like this:
ractive.merge(
"messages.stream_id1.stream",
new_message_stream,
{
compare: function ( item ) { return item.id; }
});
I also tried the compare: true option instead of the function, with the same results:
Ractive always thinks that these two messages belong effectively to the same DOM element, even though they live in a completely different message stream:
ractive.data.messages[ "stream_id1" ].stream[1].message
ractive.data.messages[ "stream_id2" ].stream[1].message
Problems:
When there are intro/outro animations ractive animates always just the end of the messages stream, even when a message in the middle of the stream was deleted, i need help to make ractive understand which messages are identical.
When i change the current_stream_id, ractive does not rerender the complete {{>render_message_stream}} partial, but goes inside the existing dom and changes the {{message}} field in all existing messages, though this might be good for dom element reuse, this triggers a lot of animations that are wrong. (Eg. it triggers intro/outro animations for the last message in the stream if stream1 has one message more than stream2).
One of these issues has a straightforward answer; unfortunately the other one doesn't.
I'll start with the easy one - the fact that
ractive.data.messages[ "stream_id1" ].stream[1].message
ractive.data.messages[ "stream_id2" ].stream[1].message
belong to the same DOM element. You're correct in that Ractive updates the existing elements rather than removing them and creating new ones - this is a core part of its design. In this case that's undesirable behaviour, but you can work around it like so:
// instead of immediately switching to a new stream ID like this...
ractive.set( 'current_stream_id', 'stream_id2' );
// you can set it to a non-existent ID. That will cause the existing DOM
// to be removed. When you set it to an ID that *does* exist, new DOM
// will be created:
ractive.set( 'current_stream_id', null );
ractive.set( 'current_stream_id', 'stream_id2' );
// or, if you'd like the initial transitions to complete first...
ractive.set( 'current_stream_id', null ).then(function () {
ractive.set( 'current_stream_id', 'stream_id2' );
});
The other issue - that merge() isn't merging, but is instead behaving as though you were doing ractive.set('messages.stream_id1.stream', new_message_stream) - is tougher. The problem is that while you and I know that {{#messages[ current_stream_id ]}} equates to messages.stream_id1 when current_stream_id === 'stream_id1, Ractive doesn't.
What it does know is that we have an expression whose value is determined by messages and current_stream_id. When the value of either of those references changes, the expression is re-evaluated, and if that value changes, the DOM gets updated - but using a standard set(). When you do ractive.merge('messages.stream_id1.stream', ...), Ractive updates all the things that depend on keypaths that are 'upstream' or 'downstream' of messages.stream_id1.stream - which includes messages. So that's how the expression knows that it needs to re-evaluate.
It's possible that a future version of Ractive will be able to handle this case in a smarter fashion. Perhaps it could make a note of arrays that are subject to merge operations, and check evaluator results to see if they're identical to one of those arrays, and if so use merge() rather than set(). Perhaps it could analyse the function in some way to see if the {{#messages[ current_stream_id ]}} section should register itself as a dependant of messages.stream_id1 for as long as current_stream_id === 'stream_id1', rather than the internally-generated ${messages-current_stream_id-} keypath.
None of that helps you in the meantime though. The only way to use merge() in your current situation is to have a separate reference that doesn't use an expression, and a bit of magic with pattern observers:
main_template:
{{#current_messages}} <!-- rather than `messages[ current_stream_id ]` -->
{{>render_message_stream}}
{{/current_messages}}
render_message_stream:
{{#current_message_stream}} <!-- rather than `stream` -->
<div class="stream">
{{>render_message}}
</div>
{{/current_message_stream}}
code:
ractive.observe( 'current_stream_id', function ( id ) {
var current_messages = this.get( 'messages.' + id );
this.set( 'current_messages', current_messages );
// hide existing stream, then show new stream
this.set( 'current_message_stream', null ).then(function () {
this.set( 'current_message_stream', current_messages.stream );
});
});
// when ANY message stream changes, we see if it's the current one - if so, we
// perform a merge on the top-level `current_message_stream` array
ractive.observe( 'messages.*.stream', function ( new_stream, old_stream, keypath, id ) {
// the value of any * characters are passed in as extra arguments, hence `id`
if ( id === this.get( 'current_stream_id' ) ) {
this.merge( 'current_message_stream', new_stream, {
compare: function ( item ) {
return item.id;
}
});
}
});
I've set up a JSFiddle demonstrating this. I hope it makes sense, let me know if not - and sorry I didn't get round to answering this question much sooner.