Horizon: Check Existence

[hnordt]

Hello.

I tried to use fetch() to check for existence, but it doesn't worked, any ideas?

// Doesn't work, neither callback dispatches
calls.find({ from: 'hnordt' }).fetch()
  .subscribe(r => console.warn(r), e => console.error(e))

[danielmewes]

A work-around is using the final callback with a local variable like this:

let foundCall = false;
calls.find({ from: 'hnordt' }).fetch()
  .subscribe(() => { foundCall = true; console.log("Call existed"); }, (e) => console.error(e), () => {
    if (!foundCall) {
      console.log("Call did not exist");
    }
  });

We should come up with an API for providing a nicer way of detecting non-existence.

Maybe find should call the result callback with null if no row exists?

[hnordt]

@danielmewes that would be what I expected initially. Awesome.

[deontologician]

I think this is the best way to do it. We could potentially return null, but I think this works better with observable operators. For example, if you do

require('rxjs/add/operator/isEmpty')

calls.find({ from: 'hnordt' }).fetch().isEmpty().subscribe(
 empty => console.log('It was', empty? ' ' : ' not', 'empty')
)

[danielmewes]

Oh I forgot there were all those nice Observable operators.

One point in favor of generating a null is that it makes it easier to throw an error if a document _should_ exist but doesn't.

@deontologician Is there an option to "split" an Observable stream? Like what if I want to use the result, but do something different if it's empty?

[marshall007]

@danielmewes .partition() will split an observable sequence into two observable streams based on some condition. There's also .defaultIfEmpty() which is probably the way to go in this case.

[danielmewes]

Cool, defaultIfEmpty() sounds like what I wanted. We should probably add an example with that do our docs.

@hnordt Does that satisfy your use case?

[hnordt]

@danielmewes yes, thank you!