I have two objects Chat and messages both in Realm database. I am fetching from db and getting two observables. Message object have chatId through which I can get chat object.
chats() - gives Observable<[Chat>
messages() - gives Observable<[Messages>
The problem is when I use map function on messages(), I get chatId, now I need chat array to filter the chatId and get chat object.
How can I do that? Can I combine both observables and get emitted both array in single closure. If yes, then how?
Depending on the context, you want to use either combineLatest or withLatestFrom.
I don't quite understand the context because you say you have an Observable<[Message]>, but you are getting a single chatID from that array. I would expect you to get an array of chat IDs.
Related
I'm writing a graphQL API using Apollo server and I would like to know is there a way to add an error to the errors array returned in the response, without throwing an actual error and failing the entire resolver.
In my code I iterate an input array and in case I encounter an unsupported item, I wish to add it to the errors array returned in the response, while still returning data for the other valid items.
Please advise if and how I can do that.
Thanks in advance.
I have a theoretical question. As I know subscription parameters must exist as a field in the returning type of the mutation. This means that the type of parameter must also match the type of the field in the returning object of the mutation. Am I right? Suppose I get an array with channels ids in the mutation response. I only send one channel id as a parameter in the subscription. Is it possible to map a subscription parameter to an array at the mutation output? If the channel id exists in the array (field channelsIds), the subscription must work. Is it possible to write this logic in the scheme itself, or is it technically impossible?
GraphQL schema:
schema {
mutation: Mutation
subscription: Subscription
}
type Mutation {
testMutation(input: TestMutationInput): TestMutationOutput
}
type TestMutationOutput {
channelsIds: [String!]!
userId: String!
userEmail: String
userPhoneNumber: String
}
type Subscription {
watchTestMutation(channelId: String!): TestMutationOutput
#aws_subscribe(mutations: ["testMutation"])
}
If I understand you correctly you want to filter based on if the mutation's returned value is in an array that is passed as an argument to the subscription. Sorry to say that is not possible at this time. Subscription filters only evaluate to true or false and cannot accommodate any logic other than that.
At the end of October 2020, I contacted AWS support for advice on this issue. I think this answer may be useful to someone, so I post their answer.
Please allow me to inform you that the use-case that you have
mentioned in the case is currently not possible via AppSync. I
understand that the lack of the feature may be causing inconvenience.
There is an internal feature request already with the AppSync team to
incorporate this feature and I have added a +1 on your behalf. It is
worth noting, that once this feature request is with the team, it will
be up to the team as to if/when this potential infrastructure feature
is implemented, and because of the limited visibility into the
progress of internal development processes, I won’t be able to provide
an ETA regarding its release. I would request you to keep an eye on
the what's new page or the AWS Blogs as all new feature requests and
enhancements are posted there[1-3].
However we can suggest a couple of workarounds in this case:
Filter the required fields on client side itself after receiving the values on the client-side from AppSync.
If the values to be filtered are very limited we can use a fake mutation made with the help of a resolver mapped to “None” Data
source. In this flow, we would create a lambda function that uses a
DynamoDB stream as the trigger. The Lambda function is triggered
whenever there's an update to the DynamoDB table.
We can then include logic in the Lambda function to filter the
required fields and perform a mutation to AppSync. In AppSync, the
mutation which was called by lambda would configured using a resolver
mapped to a “None” Data source. The None data source type passes the
request mapping template directly to the response mapping template.
And when we subscribe to this mutation, we will directly get the
filtered data from Lambda that was used to call this mutation. Please
refer to [4] for a step-by-step description of this process.
But please note that this workaround is cumbersome and would require a lot of changes if the required field values keep changing. Workaround 1(handling it on the client-side) is usually the preferred way to handle this use-case.
Resources:
[1] https://blogs.amazon.com/
[2] https://aws.amazon.com/new/
[3] https://aws.amazon.com/releasenotes/
[4] https://aws.amazon.com/premiumsupport/knowledge-center/appsync-notify-subscribers-real-time/
I want to perform an action based to the result of an asynchronous NGXS action.
In a Angular frontend app I'm using NGXS for state management. Some of the actions involve talking to a backend via REST calls. Those actions are implemented as asynchronous actions, with the reducer functions in my state classes returning an Observable.
What I'm looking for is a way to get hands on the result of the backend call, to be able to perform some action.
One use case I'm trying to implement is navigation to just created objects: Business objects are created in the frontend (Angular) app with a couple of domain properties. They get persisted in the backend, and as a result an ID for this object is created and returned to the frontend, and incorporated into the NGXS store. As a direct response to this, I'd like to navigate to a detail view for the new object. To do so, I need
(a) the information that the call has been returned successful, and
(b) the answer from the backend (the ID in this case).
Another slightly more complicated use case is the assignment of a number of tags to an business object. The tags are entities by themselfes, and have an ID each. In the UI, the user can either pick existing or add new tags. Either way, multiple tags can be added in a single step in the UI, which means I have to
call the backend for each new tag to create the ID
after all missing tags are created, update the business object with the list of tag IDs
In general, there are use cases in the frontend that depend on the result of a backend call, and there is no clean way to find this result in the store (although it's in there)
I know I can subscribe to the Observable returned from the store's dispatch method (as shown in asynchronous actions).
I also know about action handlers. In both cases I can attach code to the event of an action finished, but neither option enables me to get the result of the backend call. In the fist case, the Observable carries the whole store, while in the latter case I get the original Action, which is unfortunately missing the essential information (the ID).
The part you're missing here are selectors. Dispatching actions is not supposed to give you back a result. The only purpose of the Observable returned by store.dispatch() is to tell you when the action's handlers are done.
To get to the data returned by your calls to the backend, you have to patch the state inside your action handler. And then, outside of your state, you can access the data using store.select() or store.selectSnapshot() depending on what you need. Your state class should look somewhat like this (untested):
#State()
export class SampleState {
#Selector(SampleState)
sampleSelector(state) {
return state.sampleObject;
}
#Action(SampleAction)
sampleAction(ctx: StateContext<any>, action: sampleAction) {
return sampleBackendCall(/* ... */).pipe(
tap((result) => {
ctx.patchState({ sampleObject: result });
})
);
}
}
Now you can access this result where ever you need using the Store. For the use case of navigating to an element after its creation, you can combine a subscription to store.dispatch() with a store.selectSnapshot() like this:
store.dispatch(new SampleAction()).subscribe(() => {
navigateTo(store.selectSnapshot(SampleState.sampleSelector));
});
Note that in this easy case a selectSnapshot is perfectly fine, as we only want to get the value we just finished writing into the state. In most cases though, you will want to use store.select() or the #Select() decorator because they return Observables which enable you to also correctly display changes in your state.
That said, I'd like to add that if saving data inside the state is not necessary for you at all, then probably NGXS is the wrong library for you in the first place and you could as well just use an ordinary angular service directly returning the result of the backend call, like suggested in the comments.
My question is similar to this one RXJS: Single Observable from dynamically created Observables (no answer there).
I want to parse some pages continuously. There is the main page where I can get a list of links of pages to parse. This list of links changes over time. Then I follow the links and parse the pages continuously as well until their link disappears from the main page and start parsing new pages as they appear on the main page.
My setup for now is as follows. I have a class that given a url can return an Observable of continuously emitting items (parses a single link from the main page). This works great. However, I want to have a "master" class that will be able to return an Observable of same items but taken from multiple pages. The problem that I have is the list of pages is changing (and so is the list of underlying Observables and I can't just use Observable.merge.
TL;DR:
I have multiple Observables that I want to merge. But this list of Observables is changing dynamically and I don't know how to handle this.
How can I approach this?
If you already have Observable of "main" page, and a function to fetch items based on this data. You can use switchMap operator for "switching" this dynamically changing list, something like:
getMainPages().pipe(
switchMap(main => getItemsFromMultiplePages(main))
)
where:
getMainPages() - return Observable of main page data
getItemsFromMultiplePages(main) - return Observable of items, created by combining(maybe using merge) data from multiple pages
In Parse there is something called:
Parse.Cloud.beforeSave
I wonder if there is something to play the role of a:
Parse.Cloud.beforeRead
I need a way to control what is going to be returned to the user when a request is made to the DB.
In particular in certain circomstances, depending on information on the server, I want to force blank fields in the result of the DB request made by the user. Any standard way to do this?
There is no Parse.Cloud.beforeRead kind of function supported by Parse.
Instead, you can define a custom cloud function using
Parse.Cloud.define('readObjects', function(request, response) {...} );
that returns array of objects. This function will act as a wrapper over the Parse query.
Then, your client apps should be calling this cloud function to fetch objects rather than direct Parse.Query requests.