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()
}
}
I want to keep a list with the ID of the users who liked the specific object. To achieve that I created an array where I'm trying to keep that list.
I also want to display how many users like that object.
In mapper:
likes = getLikedListSize(it["userLikedOfferList"].toString())
then
private fun getLikedListSize(userList: String): String {
return userList.length.toString()
}
The problem is that function returns random numbers. For example in the array are two items, function return "8" etc.
What is a better approach to store list and get the size of it in Firestore?
When you are using the following method call:
getLikedListSize(it["userLikedOfferList"].toString())
It means that you are trying to pass to the getLikedListSize() method, the String representation of the array object. This representation is nothing else but the address of the object in the memory. This address it's a String that consists of 8 characters. That's the reason why, when you call .length you return the length of that String and not the actual length of the array. To solve this, simply pass the array, without converting it to a String:
getLikedListSize(it["userLikedOfferList"])
And change the method like this:
private fun getLikedListSize(userList: Array<String>): Int {
return userList.length
}
Now, when calling this method, you'll always get the number of elements that exist in the userLikedOfferList array.
In my Android app we use couchbase lite database version 2.8.6
I run three database queries.
One item in where clause.
QueryBuilder.select(
SelectResult.property("id"),
SelectResult.property("timestamp"),
SelectResult.property("rating"))
.from(DataSource.database(database))
.where(Expression.property("type").equalTo(Expression.string(DOC_TYPE)))
In the result I see three items from database printed to a console. As expected. Format here ans below is id|timestamp|rating
4e39f79c-9e11-4aba-9fb6-95d910f46cd9|0|-2147483648
e95646ee-ba3a-4978-b2a8-5383f31be2f1|0|-2147483648
e02d0eb3-6c9b-4942-b43c-a752eefc77a8|1630525956184|2147483647
I add and() condition to where() to get all items where type = 'type' AND rating < 1
QueryBuilder.select(
SelectResult.property("id"),
SelectResult.property("timestamp"),
SelectResult.property("rating"))
.from(DataSource.database(database))
.where(Expression.property("type").equalTo(Expression.string(DOC_TYPE))
.and(Expression.property("rating").lessThan(Expression.intValue(1))
)
Result is as expected as we search everything with rating < 1, third item is filtered out.
4e39f79c-9e11-4aba-9fb6-95d910f46cd9|0|-2147483648
e95646ee-ba3a-4978-b2a8-5383f31be2f1|0|-2147483648
Finally, I want to see records where type = 'type' AND rating < 1 AND timestamp <= 1
QueryBuilder.select(
SelectResult.property("id"),
SelectResult.property("timestamp"),
SelectResult.property("rating"))
.from(DataSource.database(database))
.where(Expression.property("type").equalTo(Expression.string(DOC_TYPE))
.and(Expression.property("rating").lessThan(Expression.intValue(1))
.and(Expression.property("timestamp")).lessThanOrEqualTo (Expression.longValue(1))
)
)
And now the result is really strange as I receive three items form the database. And the third one has timestamp much greater than 1 put into the query.
4e39f79c-9e11-4aba-9fb6-95d910f46cd9|0|-2147483648
e95646ee-ba3a-4978-b2a8-5383f31be2f1|0|-2147483648
e02d0eb3-6c9b-4942-b43c-a752eefc77a8|1630525956184|2147483647
Any ideas how to make it work with multiple and()? If I try to remove the second and() and keep the third one everything works as expected.
After deep investigations I found several problems in my code:
.and() should be called at the end of the "child" condition and not at the "parent" level.
For the conditions like this
val condition1 = Expression.property("type").equalTo(Expression.string(DOC_TYPE))
val condition2 = Expression.property("rating").lessThan(Expression.intValue(1))
val condition3 = Expression.property("timestamp")).lessThanOrEqualTo (Expression.longValue(1))
The correct way is
.where(condition1.and(
condition2.and(
condition3.and(
...
)
)
)
but NOT like I've tried
.where(condition1.and(condition2)
.and(condition3)
.and(...)
)
For the creation of the object I used a code that converts an object to Map<String, Any> before saving to couchbase. Everything seemed to be OK till I realized that Long was converted to Double.
The last point, I have much complicated queries and inside one of them I accidentally used .add() instead of .and(). No comments.
Hope this will help to somebody to save some time.
I would expect that the query would be as you did originally:
.where(condition1.and(condition2)
.and(condition3)
.and(...))
I think there is an additional parenthesis here that seems to be wrong:
.and(Expression.property("timestamp"))
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 a small firebase database with a list. How can I get the last inserted value of the list?
The elements of the list have as key a date string, example:
2016_05_29 --> data
2016_05_30 --> data
2016_05_31
2016_06_01
2016_06_02
2016_06_03
2016_06_10
2016_06_11
2016_06_12
2016_06_13
2016_06_14
2016_06_15
2016_06_16
2016_06_17
2016_06_18
2016_06_19 //returned value
2016_06_20
2016_06_21
2016_06_22
2016_06_23 //expected value
I am try to get the last value of this list, so 2016_06_23 but for some reason it returns this 2016_06_19 whatever key/order system I use:
Query dateMaxRef = ref.orderByKey().limitToLast(1); //this returns always 2016_06_19 instead of 2016_06_23
dateMaxRef.addListenerForSingleValueEvent(new ValueEventListener()
....
Any idea?
I faced the same issue and for me the reason was because the query was not using the latest data from firebase and instead relying on internal cache. Using .keepSynced(true) on the query fixed it for me. Below is the sample code -
Query causeListPath = mFirebase.child(PATH_CAUSE_LIST).child(SELECTED_CITY)
.child(SELECTED_COURT_CODE).limitToLast(1);
// If user wants un-cached data, force sync
if (!useCache) causeListPath.keepSynced(true);
causeListPath.addChildEventListener(new ChildEventListener() {
...
});