I want to pass an n-number of cypher-queries to a neo4j-transaction and I am thinking about a good approach.
At the moment I have a working approach that takes the array-item or if it is not available a dummy-query. (Code below)
I believe this is not best-practice. Does anybody know or have an idea how this can be done better?
function Neo4jTransaction(QueryArray) {
const session = driverWrite.session();
const tx = session.beginTransaction();
tx.run(QueryArray[0] || "RETURN 0")
tx.run(QueryArray[1] || "RETURN 0")
tx.run(QueryArray[2] || "RETURN 0")
tx.run(QueryArray[3] || "RETURN 0")
.then(result => {
return tx.commit()
}).then(() => {
session.close()
driverWrite.close()
}).catch(exception => {
console.log(exception)
session.close()
driverWrite.close()
})
}
First, if you have an array, you might want to iterate over it. Second, tx.run() returns a Promise that you need to catch if it fails. In your code, it is called 4 times in a row, but only the last one waits for the result and catches the error. I looks like some lines of the code are missing.
neo4j-driver documentation gives a good example on explicit transactions: https://github.com/neo4j/neo4j-javascript-driver#explicit-transactions
The queries are executed sequentially. If one fails the whole transaction will be rolled back.
async function neo4jTransaction(queryArray) {
const session = driver.session();
const txc = session.beginTransaction();
try {
for (const query of queryArray) {
await txc.run(query || 'RETURN 0');
}
await txc.commit();
} catch (e) {
await txc.rollback();
return Promise.reject(e);
} finally {
await session.close();
}
}
Related
I'm trying to transform a Stream of a list of one type into a Stream of a list of another type, and having an issue with this.
I have this list of Habits that I'm streaming from Firebase, and I want to accept that stream in a function, and return a new stream that is a list of ViewModels of another type from it. But my function is returning a stream of the wrong type.
Here is my code:
Stream<List<HabitCompletionViewModel>> _getTodaysHabits(
Stream<List<Habit>> habitsStream) {
var result = habitsStream.map((habitsList) {
habitsList.map(
(habit) async {
await _getHabitCompletionsCurrent(habit);
HabitCompletion completion = habit.completions!.firstWhere(
(completion) => completion.date
.dayEqualityCheck(DateTime.now().startOfDate()));
return HabitCompletionViewModel(completion: completion, habit: habit);
},
).toList();
});
return result;
}
I am getting a compile error because the result variable is showing as type Stream<Null> when I hover over it, where I would expect it to be Stream<List<HabitCompletionViewModel>>. Any idea what I'm doing wrong?
Your outer .map call does not have a return statement which is why you are getting a Stream<Null>.
So add a return statement like so:
Stream<List<HabitCompletionViewModel>> _getTodaysHabits(
Stream<List<Habit>> habitsStream) {
var result = habitsStream.map((habitsList) {
// added return statement here
return habitsList.map(
(habit) async {
await _getHabitCompletionsCurrent(habit);
HabitCompletion completion = habit.completions!.firstWhere(
(completion) =>
completion.date.dayEqualityCheck(DateTime.now().startOfDate()));
return HabitCompletionViewModel(completion: completion, habit: habit);
},
).toList();
});
return result;
}
However the above code still has an error because it is now returning a Stream<List<Future<HabitCompletionViewModel>>> instead of the desired Stream<List<HabitCompletionViewModel>>. To solve this you can use .asyncMap instead of .map.
Stream<List<HabitCompletionViewModel>> _getTodaysHabits(
Stream<List<Habit>> habitsStream) {
var result = habitsStream.asyncMap((habitsList) {
return Stream.fromIterable(habitsList).asyncMap(
(habit) async {
await _getHabitCompletionsCurrent(habit);
HabitCompletion completion = habit.completions!.firstWhere(
(completion) =>
completion.date.dayEqualityCheck(DateTime.now().startOfDate()));
return HabitCompletionViewModel(completion: completion, habit: habit);
},
).toList();
});
return result;
}
I'm currently reading variables from a Bluetooth device. This obviously takes an undetermined amount of time, so I am using futures (This method is readCharacteristic in my code down below).
More than one read operation cannot take place at a time - if a second read operation is started while a first operation is still in progress, Flutter will throw an error.
My understanding was that chaining futures together using .then() would only allow the next statement to execute when the previous call had finished. This idea seems to be true until I try to read a third value - that is when the error is thrown, because of the overlapping read events.
Here is my code:
readCharacteristic(scanDurationCharacteristic)
.then((list) => sensorScanDuration = list[0].toDouble())
.then((_) {
readCharacteristic(scanPeriodCharacteristic)
.then((list) => sensorScanPeriod = list[0].toDouble());
}).then((_) {
readCharacteristic(aggregateCharacteristic)
.then((list) => sensorAggregateCount = list[0].toDouble());
}).then((_) {
readCharacteristic(appEUICharacteristic)
.then((list) => appEUI = decimalToHexString(list));
}).then((_) {
readCharacteristic(devEUICharacteristic)
.then((list) => devEUI = decimalToHexString(list));
}).then((_) {
readCharacteristic(appKeyCharacteristic)
.then((list) => appKey = decimalToHexString(list));
});
What is a better way to ensure that these read events will not overlap?
Although R.C Howell answer is correct, prefer using async/await keywords instead. This is much more readable and you're less likely to make an error
Future<void> scanBluetooth() async {
sensorScanDuration = (await readCharacteristic(scanDurationCharacteristic))[0].toDouble();
sensorScanPeriod = (await readCharacteristic(scanPeriodCharacteristic))[0].toDouble();
sensorAggregateCount = (await readCharacteristic(aggregateCharacteristic))[0].toDouble();
appEUI = await readCharacteristic(appEUICharacteristic).then(decimalToHexString);
devEUI = await readCharacteristic(devEUICharacteristic).then(decimalToHexString);
appKey = await readCharacteristic(appKeyCharacteristic).then(decimalToHexString);
}
If you would like to chain Futures, you must return the previous Future from within the then method of the previous Future.
The documentation says to chain like so,
expensiveA()
.then((aValue) => expensiveB())
.then((bValue) => expensiveC())
.then((cValue) => doSomethingWith(cValue));
Which is the same as,
expensiveA()
.then((aValue) {
return expensiveB();
}).then((bValue) {
return expensiveC();
}).then((cValue) => doSomethingWith(cValue));
As this applies to your case,
readCharacteristic(scanDurationCharacteristic)
.then((list) {
sensorScanDuration = list[0].toDouble();
return readCharacteristic(scanPeriodCharacteristic);
}).then((list) {
sensorScanPeriod = list[0].toDouble());
return readCharacteristic(aggregateCharacteristic);
}).then((list) {
sensorAggregateCount = list[0].toDouble());
return readCharacteristic(appEUICharacteristic);
}).then((list) {
appEUI = decimalToHexString(list));
return readCharacteristic(devEUICharacteristic);
}).then((list) {
devEUI = decimalToHexString(list));
return readCharacteristic(appKeyCharacteristic);
}).then((list) => appKey = decimalToHexString(list));
I need to load a string from a file. The following code always returns null:
static String l( String name ) {
String contents;
rootBundle
.loadString( 'i10n/de.yaml' )
.then( (String r) { contents = 'found'; print( 'then()' ); })
.catchError( (e) { contents = '#Error#'; print( 'catchError()' ); })
.whenComplete(() { contents = 'dd'; print( 'whenComplete()' ); })
;
print( 'after' );
if ( null == contents ) {
return '#null#';
}
String doc = loadYaml( contents );
return doc;
}
I have added this to the flutter: section in pupspec.yaml section:
assets:
- i10n/de.yaml
- i10n/en.yaml
The file i10n/de.yaml exists.
I'm aware, that rootBundle.loadString() is async. Therefore I appended the then() call - assuming that
(String r) { contents = 'found'; }
only gets executed if the Future returned by rootBundle.loadString() is able to return a value.
Actually, the method always returns '#null#'. Therefore, I added the print() statements, which output this:
I/flutter (22382): after
I/flutter (22382): then()
I/flutter (22382): whenComplete()
OK, obviously the future of loadString() executes later than the final print() statement.
Q: But how do I force the future to execute, so that I may retrieve its value?
In other words: How do I wrap some async stuff in certain code to retrieve its value immediately?
PS: First day of flutter/dart. Probably a trivial question...
the .then() is getting executed, but after the rest of the body. As you mention loadString() returns a Future, so completes in the future. To wait for a Future to complete use await. (Note that when you mark the function as async, the function must now return a Future itself - as it has to wait for loadString to complete in the future, so it itself has to complete in the future...) When you call l('something') you will have to await the result.
Future<String> l(String name) async {
try {
String contents = await rootBundle.loadString('i10n/de.yaml');
return contents == null ? '#null#' : loadYaml(contents);
} catch (e) {
return 'oops $e';
}
}
It's no big deal that lots of your utility functions become async as a result of having to wait for things (there's a lot of waiting - for files to read, http requests to complete, etc). You'll eventually end up at the top with something like (call this from initState)
refresh(String s) {
l(s).then((r) {
setState(() {
i18nStuff = r;
});
});
}
to set the state of your Widget when the i18nStuff is ready, and a Widget that has this in its build to switch between a dummy ui for the few milliseconds until it's ready, and then the real UI.
Widget build() {
if (i18nStuff == null) {
return new Container();
}
return new Column(
// build the real UI here
);
}
I've read several stackoverflow questions, dart documents, and even watched a video on async and await. I haven't found an answer to my question. I would like to call an async method, execute other code, and then wait on the completion of the async task.
Here's an example of what I'm working with. This is my component
Credit credit;
...
Future<Null> getCredit(id) async {
try {
credit = await _creditService.getCredit(id);
}
catch (e) {
errorMessage = e.toString();
}
}
...
void onUpdateCredit(int credit_id) {
getCredit(credit_id);
creditDialogTitle = 'Update Credit';
creditArtistIndex = credit.artist_id;
instrument = credit.instrument;
creditNotes = credit.notes;
creditDialog.open();
}
This code crashes because credit is null when an attempt is made to use it. One way around it is combine the two methods:
Future<Null> onUpdateCredit(id) async {
try {
credit = await _creditService.getCredit(id);
creditDialogTitle = 'Update Credit';
creditArtistIndex = credit.artist_id;
instrument = credit.instrument;
creditNotes = credit.notes;
creditDialog.open();
}
catch (e) {
errorMessage = e.toString();
}
}
Nothing is done is parallel and, if I need the credit some where else in my code, I would have to duplicate the try/catch portion of the method. I could also code it like this:
void onUpdateCredit(int credit_id) {
credit = null;
getCredit(credit_id);
creditDialogTitle = 'Update Credit';
while (credit == null) {//wait a period of time}
creditArtistIndex = credit.artist_id;
instrument = credit.instrument;
creditNotes = credit.notes;
creditDialog.open();
}
In other situations, I do something similar to this in my html with *ngIf="var != null" where var is populated by a future.
Is there a better way than using while (credit == null) ? This example only executes one instruction between the request and the completion so is trivial. I'm sure I'll other situations where I have a lot to do in between. I'm also adding the service method:
Future<Credit> getCredit(int id) async {
try {
String url = "http://catbox.loc/credits/${id.toString()}";
HttpRequest response = await HttpRequest.request(
url, requestHeaders: headers);
Map data = JSON.decode(response.responseText);
final credit = new Credit.fromJson(data);
return credit;
}
catch (e) {
throw _handleError(e);
}
}
Update
Based on #Douglas' answer, this works:
Future<Null> onUpdateCredit(id) async {
Future future = getCredit(id);
creditDialogTitle = 'Update Credit';
await future;
creditArtistIndex = credit.artist_id;
instrument = credit.instrument;
creditNotes = credit.notes;
creditDialog.open();
}
I then eliminated the intervening method.
Future<Null> onUpdateCredit(id) async {
try {
Future<Credit> future = _creditService.getCredit(id);
creditDialogTitle = 'Update Credit';
credit = await future;
creditArtistIndex = credit.artist_id;
instrument = credit.instrument;
creditNotes = credit.notes;
creditDialog.open();
}
catch (e) {
errorMessage = e.toString();
}
}
getCredit(credit_id) does not just kick off an asynchronous call, it also returns a Future object - immediately. Store that object in a local variable, and you can use it later to asynchronously execute additional code when it completes.
There are two ways to use that Future object. The easier and more fluent way requires that you declare onUpdateCredit to be async. Inside an async function, the line await futureObject will cause all code after that line to be executed asynchronously after the Future completes. The complete version of onUpdateCredit using this technique would look like this:
Future<Null> onUpdateCredit(int credit_id) async {
Future future = getCredit(credit_id);
creditDialogTitle = 'Update Credit';
await future;
creditArtistIndex = credit.artist_id;
instrument = credit.instrument;
creditNotes = credit.notes;
creditDialog.open();
}
The other way is to explicitly register the rest of your code as a callback using .then(). That would look like this:
void onUpdateCredit(int credit_id) {
Future future = getCredit(credit_id);
creditDialogTitle = 'Update Credit';
future.then((_) => {
creditArtistIndex = credit.artist_id;
instrument = credit.instrument;
creditNotes = credit.notes;
creditDialog.open();
});
}
Note that in either case, if the exception path occurs in getCredit(id) you will get errors for credit not being set. If you truly want the exception to be swallowed silently, you should have its handler fill in a default value for credit so that code that assumes it completed normally will still work.
Also note that your while loop version would fail - Dart, like JavaScript, is not truly multithreaded, and busy waiting like that will block the event loop forever, preventing the code that would set credit from ever running.
A short summary of how async and await work in general:
Future someFunc(args) async {
...
return value;
}
is equivalent to:
Future someFunc(args) {
return new Future(() => {
...
return value;
}
}
The code inside gets executed on a later iteration of the event loop, and the returned future completes either successfully with value or exceptionally with anything thrown in that code.
Meanwhile this:
try {
value = await someFutureObject;
...more code here...
} catch (e) {
...exception handling here...
}
is equivalent to:
someFutureObject.then((value) => {
...more code here...
}).catchError((e) => {
...exception handling here...
});
The most common use case is someVar = await someAsyncCall();, but you can save the Future itself by omitting await, or you can await on an arbitrary Future object no matter where it comes from.
However - and this is what makes await and async so incredibly convenient - you can have 5 different exit points from the async function interspersed with three await calls (in the same async function) in assorted places inside 13 nested loops, switches, try/catch, and if blocks, and Dart will automatically figure out the necessary tree of callbacks to make it all follow the same code path as if all the calls were synchronous.
Lets say i have this code:
map = %Q{
function() {
emit(this.name, { likes: this.likes });
}
}
reduce = %Q{
function(key, values) {
var result = { likes: 0 };
values.forEach(function(value) {
if(value.likes < 0){
#{Rails.logger.error "likes are negativ" }
}
result.likes += value.likes;
});
return result;
}
}
Band.where(:likes.gt => 100).map_reduce(map, reduce).out(inline: true)
As you can see I want to record an error if the value.likes are negativ:
#{Rails.logger.error "likes are negativ" }
But this Code is executed each time I run the aggregate and not when the likes are negativ.
What can I do to throw an error in the aggregate statement?
Lets just analyze the code. Firstly:
map = %Q{
function() {
emit(this.name, { likes: this.likes });
}
}
Here a string is assigned to a variable map. Please not the %Q{} is just another way of writing "". Former is another syntax to easily define strings which have a double quote. e.g.
# pretty
%Q{He said "You are awesome"}
# not so pretty
"He said \"You are awesome\""
Next there is:
reduce = %Q{
function(key, values) {
var result = { likes: 0 };
values.forEach(function(value) {
if(value.likes < 0){
#{Rails.logger.error "likes are negative" }
}
result.likes += value.likes;
});
return result;
}
}
Here another string is assigned to a variable reduce. #{Rails.logger.error "likes are negative" } is just a regular string interpolation logging an error and returning true. So above code is equivalent to:
Rails.logger.error "likes are negative"
reduce = %Q{
function(key, values) {
var result = { likes: 0 };
values.forEach(function(value) {
if(value.likes < 0){
true
}
result.likes += value.likes;
});
return result;
}
}
You see why the logging statement is executed every time.
Next there is:
Band.where(:likes.gt => 100).map_reduce(map, reduce).out(inline: true)
This is just a simple statement using mongoid to execute a map-reduce command on the mongo server, passing map and reduce functions constructed earlier.
Note that in above code, intention is to execute ruby code in a javascript reduce function. However that is not possible, as reduce function is being executed on mongodb server and cannot execute the logging statement.
One way to handle the situation could be to reduce to a hash like {likes: 0, negative_likes: 0}, incrementing negative_likes conditionally and logging error on receiving result.
PS: it might be a better idea to use aggregation framework instead of map-reduce.