I will first say that I am a firmware developer trying to make a web interface for a personal project. My question might be basic as this is my first web interface with a database. I have searched quite a bit on how to achieve what I am trying to do without success, so I suspect I am not approaching this the right way, but here it is:
Basically I have a device pushing data to a firebase realtime database :
the push function generates an unique Id for me automatically which is nice...
Now In my flutter interface, I want to display the latest entry on a widget so I am using the onChildAdded functionnality of flutterfire:
ref.onChildAdded.listen((DatabaseEvent event) {
print(event.snapshot.key);
print(event.snapshot.value);
});
My first issue is that this function is triggered for all the child at first before waiting for new ones which is unnecessary and could be a problem when I begin to have a lot of data. Is there a way to simply get the latest sample and still get an event when one is added? (I still want to keep the old data to be able to make charts with them)
My second problem is the format of the received data:
{
-Mx2PCeptLf2REP1YFH0: {
"picture_url": "",
"time_stamp": "2022-02-28 21:56:58.502005",
"temperature": 27.71,
"relative_humidity": 42.77,
"eco2": 691,
"tvoc": 198,
"luminosity": 4193,
"vpd": 1.71
}
}
before using the put method, I didn't have the automatically generated ID so I was able to cast this as a Map<String, dynamic> and use the data like this in my widget:
Text(snapshot.data!["temperature"].toString())
but now the added nesting with the generated id is giving me a headache as I can't seem to figure out how to simply get the data.
So if anyone could help me to always get the single latest data when subscribing to a collection and how to access that data within my State of my StatefulWidget that would be much appreciated!
My first issue is that this function is triggered for all the child at first before waiting for new ones which is unnecessary and could be a problem when I begin to have a lot of data.
The Firebase Realtime Database synchronizes the state of the path/query that you listen to. So it doesn't just fire an event on onChildAdded for new data, but also for any existing data that you request. There is (intentionally) no way to change that behavior.
So your options here are limited:
You can remove the data that your application has processed, so that it doesn't get passed to the clients again. That means you essentially implement a message-passing mechanism on top of Firebase's data-synchronization semantics.
You can use a query to only request a certain subset of the child nodes at a path. For example, if you remember the latest key that you've received, you can get data from there on with ref.orderByKey().startAt("-Mx2LastKeyYouAlreadySaw").
Since this comes up regularly, I recommend also checking:
How can I setup Firebase to observe only new data?
How to get only updated data Firebase
How to fetch only latest specific data from Firebase?
and more from these search results
My second problem is the format of the received data:
The screenshot of your database shows two keys, each of which has a single string value. The contents of that value may be JSON, but the way they are stored is just a single string.
If you want to get a single property from the JSON, you either have to:
Decode the string back into JSON with jsonDecode.
Fix the problem at the source by storing the data as proper JSON, rather than as a string.
Related
What would a Cosmos stored procedure look like that would set the PumperID field for every record to a default value?
We are needing to do this to repair some data, so the procedure would visit every record that has a PumperID field (not all docs have this), and set it to a default value.
Assuming a one-time data maintenance task, arguably the simplest solution is to create a single purpose .NET Core console app and use the SDK to query for the items that require changes, and perform the updates. I've used this approach to rename properties, for example. This works for any Cosmos database and doesn't require deploying any stored procs or otherwise.
Ideally, it is designed to be idempotent so it can be run multiple times if several passes are required to catch new data coming in. If the item count is large, one could optionally use the SDK operations to scale up throughput on start and scale back down when finished. For performance run it close to the endpoint on an Azure Virtual Machine or Function.
For scenarios where you want to iterate through every item in a container and update a property, the best means to accomplish this is to use the Change Feed Processor and run the operation in an Azure function or VM. See Change Feed Processor to learn more and examples to start with.
With Change Feed you will want to start it to read from the beginning of the container. To do this see Reading Change Feed from the beginning.
Then within your delegate you will read each item off the change feed, check it's value and then call ReplaceItemAsync() to write back if it needed to be updated.
static async Task HandleChangesAsync(IReadOnlyCollection<MyType> changes, CancellationToken cancellationToken)
{
Console.WriteLine("Started handling changes...");
foreach (MyType item in changes)
{
if(item.PumperID == null)
{
item.PumperID = "some value"
//call ReplaceItemAsync(), etc.
}
}
Console.WriteLine("Finished handling changes.");
}
i am trying to update my firebase but is not appear what i updated
this is my firebase data :
and then i exported my data :
then i updated and added another class :
and return a message is all updated :
but the problem is my data don't updated and stay in same data :
and this is my rules :
please help me to know what is and why i cant success to update that
thanks alot
Firebase automatically creates keys for any values you write, and it automatically deletes keys that no longer have any values under them.
Since you specified no value for the game key, it is immediately deleted after you add it.
If you want to make sure the key gets written when you import the JSON, be sure to give it a value. E.g.
"game": true
But note that you don't need to pre-create keys like this, as Firebase will automatically create the game key when your app writes a value somewhere under it.
I'm currently creating a dashboard application for my main application, this dashboard is able to display in charts the demography of the users that uses the app. I use Firebase Database as the backend. The JSON tree of my DB is as shown below. My question is, how do I get the amount of data with a specific value of a key? Example: the number of children with the value 'Pria' for the key 'jk' is 2.
My Backend JSON Tree:
So far, I'm able to get all of the data using:
DatabaseReference itemRef = FirebaseDatabase.instance.reference().child('data_pengguna');
And I've also tried the codes below, but it doesn't seem to work:
int jmlPria;
FirebaseDatabase.instance
.reference()
.child('data_pengguna')
.orderByChild('jk')
.equalTo('Pria')
.once()
.then((onValue) {
Map data = onValue.value;
jmlPria = data.length;
});
But I haven't successfully filtered the data and put it inside a variable, can anyone help me?
Many thanks in advance.
That last snippet looks correct, jmlPria should have the number of children.
But the value of jmlPria will only be set to the latest value inside the then() callback. Make sure that Text($jmlPria) is inside the then() callback. Outside of that, jmlPria will not have the correct value.
Also see Doug's great blog post on asynchronous programming.
I have a situation in which I need to subscribe to the same collection twice. The two publish methods in my server-side code are as follows:
Meteor.publish("selected_full_mycollection", function (important_id_list) {
check(important_id_list, Match.Any); // should do better check
// this will return the full doc, including a very long array it contains
return MyCollection.find({
important_id: {$in: important_id_list}
});
});
Meteor.publish("all_brief_mycollection", function() {
// this will return all documents, but only the id and first item in the array
return MyCollection.find({}, {fields: {
important_id: 1,
very_long_array: {$slice: 1}
}});
});
My problem is that I am not seeing the full documents on the client end after I subscribe to them. I think this is because they are being over-written by the method that publishes only the brief versions.
I don't want to clog up my client memory with long arrays when I don't need them, but I do want them available when I do need them.
The brief version is subscribed to on startup. The full version is subscribed to when the user visits a template that drills down for more insight.
How can I properly manage this situation?
TL/DR - skip to the third paragraph.
I'd speculate that this is because the publish function thinks that the very_long_array field has already been sent to the client, so it doesn't send it again. You'd have to fiddle around a bit to confirm this, but sending different data on the same field is bound to cause some problems.
In terms of subscribing on two collections, you're not supposed to be able to do this as the unique mongo collection name needs to be provided to the client and server-side collections object. In practice, you might be able to do something really hacky by making one client subscription a fake remote subscription via DDP and having it populate a totally separate Javascript object. However, this cannot be the best option.
This situation would be resolved by publishing your summary on something other than the same field. Unfortunately, you can't use transforms when returning cursors from a publish function (which would be the easiest way), but you have two options:
Use the low-level publications API as detailed in this answer.
Use collection hooks to populate another field (like very_long_array_summary) with the first item in the array whenever very_long_array changes and publish just the summary field in the former publication.
A third option might be publishing the long version to a different collection that exists for this purpose on the client only. You might want to check the "Advanced Pub/Sub" Chapter of Discover Meteor (last sub chapter).
On my meteor project users can post events and they have to choose (via an autocomplete) in which city it will take place. I have a full list of french cities and it will never be updated.
I want to use a collection and publish-subscribes based on the input of the autocomplete because I don't want the client to download the full database (5MB). Is there a way, for performance, to tell meteor that this collection is "static"? Or does it make no difference?
Could anyone suggest a different approach?
When you "want to tell the server that a collection is static", I am aware of two potential optimizations:
Don't observe the database using a live query because the data will never change
Don't store the results of this query in the merge box because it doesn't need to be tracked and compared with other data (saving memory and CPU)
(1) is something you can do rather easily by constructing your own publish cursor. However, if any client is observing the same query, I believe Meteor will (at least in the future) optimize for that so it's still just one live query for any number of clients. As for (2), I am not aware of any straightforward way to do this because it could potentially mess up the data merging over multiple publications and subscriptions.
To avoid using a live query, you can manually add data to the publish function instead of returning a cursor, which causes the .observe() function to be called to hook up data to the subscription. Here's a simple example:
Meteor.publish(function() {
var sub = this;
var args = {}; // what you're find()ing
Foo.find(args).forEach(function(document) {
sub.added("client_collection_name", document._id, document);
});
sub.ready();
});
This will cause the data to be added to client_collection_name on the client side, which could have the same name as the collection referenced by Foo, or something different. Be aware that you can do many other things with publications (also, see the link above.)
UPDATE: To resolve issues from (2), which can be potentially very problematic depending on the size of the collection, it's necessary to bypass Meteor altogether. See https://stackoverflow.com/a/21835534/586086 for one way to do it. Another way is to just return the collection fetch()ed as a method call, although this doesn't have the benefits of compression.
From Meteor doc :
"Any change to the collection that changes the documents in a cursor will trigger a recomputation. To disable this behavior, pass {reactive: false} as an option to find."
I think this simple option is the best answer
You don't need to publish your whole collection.
1.Show autocomplete options only after user has inputted first 3 letters - this will narrow your search significantly.
2.Provide no more than 5-10 cities as options - this will keep your recordset really small - thus no need to push 5mb of data to each user.
Your publication should look like this:
Meteor.publish('pub-name', function(userInput){
var firstLetters = new RegExp('^' + userInput);
return Cities.find({name:firstLetters},{limit:10,sort:{name:1}});
});