My top level state model looks like this:
{
listOfItems: [], // Item[]
selections: {
itemId: 0
}
}
The items list may contain 10 different shopping items.
When the user clicks on an item, it updates the selection, and my #Selector will rerun.
Action: Set Item Selection
#Action(Item.Select)
setState(
{ setState }: StateContext<ItemsModel>,
{ itemId }: Item.Select
) {
setState(patch({ selections: patch({ itemId }) }));
}
Selector: Select Current Item
#Selector()
static getSelectedItem(state: ItemModel): Item {
return state.itemList.find(i => i.itemId === state.selections.itemId);
}
Problem is: I have up to 20 actions to perform on the selected Item. This results in:
Lots of .find() lookups to find item in the original array (both selector and actions)
Actions to perform on the listOfItems are in the same place as those to perform on a specific Item
I would like to: Keep the array and selection in this state, but separate out the "selected item" into a new substate, where the child state's model can just be Item type. This way I can encapsulate all the actions on Item in a different place to actions on the Items[] array.
I'm not sure how to keep them in sync. I need to keep the 'selectedItem' state up to date when the selection itemId changes in the parent. I also need to make sure any mutations to the selectedItem are reflected in the original array in the parent.
This seems like it might be more of a fundamental problem with how you are trying to represent your application state. Have you thought of normalizing your list of items? Or at least using a key/value lookup object instead of an array? You wouldn't need to use the .find() to do your lookup and could access the key of the object via the unique id you are interested in. Let me know if that is of any help!
I'm working on a Firebase Cloud Function. When I log the value of change.after.val() I get a printout of a key-value pair
{ DCBPUTBPT5haNaMvMZRNEpOAWXf3: 'https://m.youtube.com/watch?v=t-7mQhSZRgM' }
rather than simply the value (the URL). Here's my code. What am I not understanding about .val() ? Shouldn't "updated" simply contain the URL?
exports.fanOutLink = functions.database.ref('/userLink').onWrite((change, context) => {
const updated = change.after.val();
console.log(updated);
return null
});
If you want only the URL value, you should include a wildcard in your trigger path for the URL key:
exports.fanOutLink = functions.database.ref('/userLink/{keyId}').onWrite((change, context) => {
console.log('keyId=', context.params.keyId);
const updated = change.after.val();
console.log(updated);
return null
});
In the Realtime Database, data is modeled as a JSON tree. The path specified in an event trigger identifies a node in the tree. The value of the node, being JSON, includes all child nodes. The change parameter for the trigger event refers to the value of the entire node.
I indicated above that you can change the trigger path to refer one level down. An alternative is to access the children of the node using the child() method of DataSnapshot.
Without knowing your use-case, it's hard to be more specific about the trigger event path you should use. Keep in mind that the event fires when any element of the node value changes, whether it be a simple value at the root level, or a value of a child node. It is often the case that you want the trigger to be as specific as possible, to better identify what changed. That's where wildcards in the path are useful. As I showed in the code I posted, the string value of a wildcard is available from the context parameter.
Suppose i have a structure like this:
items: {
id1: {
likes: 123
},
id2: {
likes: 456
},
id3: {
sourceId: 'id1',
likes: 123
},
id4: {
sourceId: 'id1',
likes: 123
}
[,...]
}
where any item can either be a source item or an item that references a source item. The purpose of referencing the source item is to keep counters consistent across all items that share the same sourceId.
Therefore, when i change a counter on a source item, i want to batch write that counter's value to all the items that have that item as their sourceId.
My concern
Suppose I read in the docs referencing some sourceId, then i commit the changes in a batch to all of them. What if a very small subset of the docs in the batch were deleted in the small window of time since the documents were read in, or a rare but possible write-rate conflict occurs? Do none of the counters get updated because 1 or 2 documents failed to be updated?
Is it possible to submit a batch that writes the changes to each of its documents independently, such that if one fails, it has no impact on if the others get written?
Or maybe for this scenario it might be better to read in the documents referencing some sourceId and then write the changes to each document in parallel such that write independence is achieved. I don't like this approach since the number of requests would be the size of the batch.
What are your thoughts?
Take a careful read of the API docs for BatchWrite. It will answer your questions. Since you're not showing your batch code (are you using set? update?), we have to look at the API docs to assess the failure cases:
create()
This will fail the batch if a document exists at its location.
It sounds like you're probably not using create(), but this is the failure case.
set()
If the document does not exist yet, it will be created.
So, a set will not fail if the documented was deleted before the batch got committed.
update()
If the document doesn't yet exist, the update fails and the entire
batch will be rejected.
So, if you try to update a nonexistent document, the batch will fail.
If you want to decide what to do with each document, depending on its existence and contents, then control the failure cases, use a transaction.
If I understand your question, I had a similar scenario and here's how I did it. First, I use generated universal ID's, uid's, for all my item keys/id's. Then, what you do is on the grandchildren, simply write the uid of the parent it is associated with. Each grandchild could be associated with more than one parent.
As you create new items, you have to recursively update the parent with the uid of the item so the parent has record of all its associated child items.
fetchPromise = [];
fetchArray = [];
if(!selectIsEmpty("order-panel-one-series-select") || !selectIsUnselected(orderPanelOneSeriesSelect)){
orderPanelOneTitle.innerHTML = "Select " + orderPanelOneSeriesSelect.value.toString().toUpperCase() + " Option";
}
//on change we need to populate option select based on this value
//now we need to get the particular series doc and fill get array then prmise all to get all options
familyuid = getRecordFromList(doc.getElementById("order-panel-one-family-select"),orderPanelOneFamilySelect.selectedIndex);
seriesuid = getRecordFromList(doc.getElementById("order-panel-one-series-select"),orderPanelOneSeriesSelect.selectedIndex);
optionuid = getRecordFromList(doc.getElementById("order-panel-one-option-select"),orderPanelOneOptionsSelect.selectedIndex);
optionRef = db.collection("products").doc(familyuid).collection("option");
itemRef = db.collection("products").doc(familyuid).collection("item");
targetRef = db.collection("products").doc(familyuid).collection("option").doc(optionuid);
try {
targetRef.get().then(function(snap) {
if (snap.exists) {
for (var key in snap.data()) {
if (snap.data().hasOwnProperty(key)) {
fetchPromise = itemRef.doc(key).get();
fetchArray.push(fetchPromise);
}
}
Promise.all(fetchArray).then(function(values) {
populateSelectFromQueryValues(values,"order-panel-one-item-select");
if(!selectIsEmpty("order-panel-one-item-select")){
enableSelect("order-panel-one-item-select");
}
targetRef.get().then(function(snap){
if(snap.data().name){
var str = snap.data().name.toString();
orderAsideInfo.innerHTML = "Select " + capitalizeFirstLetter(str) + " item.";
}
});
});
}
}).catch(function(error) {
toast("error check console");
console.log("Error getting document:", error);
});
}
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 the following client-side code (with two collections A and B):
var id = A.insert({name: 'new element of A');
var b = B.findOne({name: 'parent of new element of A'});
B.update(b._id, { $set: { child: id } });
The above code works fine, i.e., the server side collections are correctly updated. The problem happens on the client where I have a template that reacts on both A and B collection changes.
The template reacts as follows:
It immediately redraws itself, based on the latency compensation mechanism, showing the correct changes for both collections
Soon after, it redraws itself again but without the changes (as if they had been rejected)
It does not refresh automatically anymore afterwards. But, if I hit the refresh button, the template redraws once more and now shows the correctly updated collections (both A and B)
On the second refresh only one of the collection (the parent B) has been updated and the template displays incoherent data (as if the updates had not happened).
I think this is because I am not dealing here with one single transaction that updates both collections at the same time, confusing the client side template.
How can I solve this?
EDIT:
I must add that in my case I have two complementing subscriptions to the child database:
var A = new Meteor.Collection('children');
handle1 = Meteor.subscribe('children1');
handle2 = Meteor.subscribe('children2');
and on the server
Meteor.publish('children1', function () {
return A.find({ sex: male });
}
Meteor.publish('children2', function () {
return A.find({ sex: female });
}
Could this be the reason for, when I insert a new element in A, I get the weird behavior described above?
it might work better if you do it all in one command
B.update({name: 'parent of new element of A'}, { $set: { child: A.insert({name: 'new element of A') } });
you'll have to put this in a method since you arent updating based on the id
you could also try to manually call Deps.flush() after the operations, that shouldnt be needed, but its something to try