I am reading this piece of code
https://coryrylan.com/blog/angular-observable-data-services
esp this line
this._todos.next(Object.assign({}, this.dataStore).todos);
What i don't understand is why after the call to
Object.assign({}, this.dataStore ) which copies the contents of the datastore to an new object why not do just
this._todos.next(Object.assign({}, this.dataStore))
instead of
this._todos.next(Object.assign({}, this.dataStore).todos)
Any ideas ?
Because it is good practice to expose only the data that a component requires to that component.
The following line causes the _todos Subject to only emit the Todos data:
this._todos.next(Object.assign({}, this.dataStore).todos)
The code below, which you suggest, would cause the _todos subject to emit the entire dataStore:
this._todos.next(Object.assign({}, this.dataStore))
Related
What I am really interested in with data-loader is the per request caching. For example say my graphql query needs to call getUser("id1") 3x. I would like something to dedupe that call.
However it seems like with data-loader I need to pass in an array of keys into my batch function, and multiple requests will be batched into one api calls.
This has me making a few assumptions that i dont like:
1.) That each service I am calling has a batch api (some of the ones im dealing with do not).
2.) What if multiple calls get batched into 1 api call, and that call fails because 1 of the items was not found. Normally I could handle this by returning null for that field, and that could be a valid case. Now however my entire call may fail, if the batch API decides to throw an error since 1 item was not found.
Is there anyway to use dataloader with single-key requests.
Both assumptions are wrong because the implementation of the batch function is ultimately left up to you. As indicated in the documentation, the only requirements when writing your batch function are as follows:
A batch loading function accepts an Array of keys, and returns a Promise which resolves to an Array of values or Error instances.
So there's no need for the underlying data source to also accept an array of IDs. And there's no need for one or more failed calls to cause the whole function to throw since you can return either null or an error instance for any particular ID in the array you return. In fact, your batch function should never throw and instead should always return an array of one or more errors.
In other words, your implementation of the batch function might look something like:
async function batchFn (ids) {
const result = await Promise.all(ids.map(async (id) => {
try {
const foo = await getFooById(id)
return foo
} catch (e) {
// either return null or the error
}
}))
}
It's worth noting that it's also possible to set the maxBatchSize to 1 to effectively disable batching. However, this doesn't change the requirements for how your batch function is implemented -- it always needs to take an array of IDs and always needs to return an array of values/errors of the same length as the array of IDs.
Daniel's solution is perfectly fine and is in fact what I've used so far, after extracting it into a helper function.
But I just found another solution that does not require that much boilerplate code.
new DataLoader<string, string>(async ([key]) => [await getEntityById(key)], {batch: false});
When we set batch: false then we should always get a key-array of size one passed as argument. We can therefore simply destructure it and return a one-sized array with the data. Notice the brackets arround the return value! If you omit those, this could go horribly wrong e.g. for string values.
I'm working with socket.io. I am having trouble receiving data from the server even though I can console.log() the data (an array of objects) right before I try to emit the data back to the calling client. If I hard code info into the emit, it will display on the client but when I use the dynamically created array of objects, it doesn't go through. My gut tells me its a asyn issue but I'm using bluebird to manage that. Perhaps its an encoding issue? I tried JSON.stringify and JSON.parse, no dice.
Server
socket.on('getClassList', function(){
sCon.getClassList()
.then(function(data){
console.log(data) //data is an array full of objects
socket.emit('STC_gotDatList', data)
})
})
Hard-Coded Expected Output:
classes['moof'] = {
accessList: [888],
connectedList: [],
firstName: "sallyburnsjellyworth"
}
Client
socket.on('STC_gotDatList', function(info){
console.log(info) //prints [] or {}
})
EDIT:
I remember reading somewhere that console.log() may not be printing data at the time the data is available/populated. Could that be it even though im using Promises? At the time I'm emitting the data, it just hasn't been populated into the array? How would i troubleshoot this scenario?
EDIT2:
A step closer. In order to get anything to return, for some reason I have to return each specific object in the 'classes' array. It wont allow me to send the entire array, for the example I gave above, to get data to the client, I have to return(classes['moof']), can't return(classes) to get the entire array... Not sure why.
EDIT3: Solution: You just can't do it this way. I had to put 'moof' inside classes as a property (className) and then I was able to pass the whole classes array.
How is the 'classes' array created?
The problem might be that it is being given properties dynamically, but has not been created as an object, but rather an array.
If you plan to dynamically add properties to it (like property moof), you should create it as an object (with {}) rather than an array (with []).
Try
var classes = {};//instead of classes = []
//then fill it however you do it
classes[property] = {
accessList: [888],
connectedList: [],
firstName: "sallyburnsjellyworth"
};
Also, the credit goes to Felix, I'm just paraphrasing his answer: https://stackoverflow.com/a/8865468/7573546
I setup a subject and then put some methods on it. It seems to work as intended until it gets to .switch() which I thought would simply keep track of the last call. I get the error Property 'subscribe' does not exist on type 'ApiChange' It seems to convert it to type ApiChange from an observable. I don't understand this behavior. Should I be using a different operator?
Service:
private apiChange = new Subject<ApiChange>();
apiChange$ = this.apiChange.asObservable().distinctUntilChanged().debounceTime(1000).switch();
Component:
this.service.apiChange$.subscribe(change => {
this.service.method(change);
});
.debounceTime(1000) will already assure you will only get a maximum of one value emitted from your observable chain per second. All the values preceding the 1 second quiet time will already be discarded.
With a simple Subject (not a ReplaySubject), past values are not provided to subscribers anyway.
You probably just want to skip the .switch() and enjoy the chain without it.
I have an Observable that emits a List of entries as below:
val obsList: Observable[List[MyEntry] = Observable.from(getListEntry)
// getListEntry would give me a List[MyEntry]
How could I now get the contents piped into another Observable that takes in a MyEntry? I mean, for each entry in the original Observable, I would need to pipe them to another Observable that has a type signature of:
Observable[MyEntry]
I figured out how to do this! I'm using the Monifu library!
So, I would be doing:
Observable.fromIterable(listOfEntry)
Someone doesn't understand what i want. I want to know what i can do with a pointer.
I have js cloud code.
I have a pointer.
What can i do with it?
Example:
var query = new Parse.Query("Messages"); //POINTER QUERY
console.log(userMessages[0].get("messageId"));
console.log("end2");
query.equalTo("objectId",userMessages[position].get("messageId"));
In the example, userMessages is the result of a prior cloud query.
This line
console.log(userMessages[0].get("messageId"));
helpfully outputs
{"__type":"Pointer","className":"Messages","objectId":"5J4eOletgz"}
This is less useful than you might imagine. I cannot seem to call the objectId from it, and the query
query.equalTo("objectId",userMessages[position].get("messageId"));
query.find ({ ... });
returns nothing. Note that the query should find the pointer-object the pointer-points-to, but instead it helpfully throws the error
Error: 102 bad special key: __type
Which is just about useless.
What can i do with a pointer?
Why don't the people at parse.com bother to write this stuff up anywhere?
That second question is more like a buddhist koan for them to meditate over, no need to respond!
you can use:
userMessagesQuery.include("messageId")
before you execute your query that returns userMessages and you will get the entire object in "messageId" instead of just a pointer.
Also you use
userMessages[0].get("messageId").fetch({success:function(){}})
to get the full object if you don't want to use "include"
Suggestion: I'd rename "messageId" to "message" to make it clear that it's an object pointer and not an ID field.
A pointer is a Parse object with just the minimal data used for linking. If you want to get the rest of the data for a pointer (fully populate it), use the fetch() method.
If you just want the objectId from a pointer, you retrieve it just like you would any other Parse object, by using the myPointer.objectId property.
In your case the following would work, but isn't the most optimal solution:
// I would suggest renaming messageId if you're actually storing pointers
var messagePointer = userMessages[position].get("messageId");
query.equalTo("objectId", messagePointer.objectId);
Instead, as stated by #RipRyness, you could just change your previous query to include() the full object, avoiding many extra queries.
userMessagesQuery.include("messageId");
// ... later in success handler ...
// now a fully populated Message object
var message = userMessages[position].get("messageId");
console.log(message);
Okay so a pointer is does not only point to a certain object, but it can BE that object.
In the query that returns the userMessages result, you can use the line .include("messageId") - this makes your query not only return a pointer to that object, it actually includes that object in place of the pointer.
After using the include statement, userMessages[0].get("messageId") will return the Message object linked that that userMessage object. You no longer need to query the Message collection to get the object.