Determine if a condition is met in Firebase Observable using scan() and return the

Within Firebase, I have objects for articles structured like this:

articles
 UNIQUE_KEY
  title: 'Some title'
  validUntil: '2017-09-29T21:00:00.000Z'
 UNIQUE_KEY
  title: 'Other title'
  validUntil: '2017-10-29T21:00:00.000Z'

Currently, I am trying to fetch only those articles whose validUntil date is in the future. This is the function I've implemented in my service:

  getValidArticles(): Observable<Article[]> {
    return this.afDb.list('articles').do(console.log)
      .scan((article) => { 
        if (new Date(article.validUntil).getTime() >= new Date().getTime()) {
          return article;
        }
      })
  }

It seems to be working fine, but I noticed that upon logging with .do(console.log), two identical objects are displayed in the console. I'm not sure if I'm approaching this correctly. Can anyone provide some insight?

angulartypescriptfirebase-realtime-databaseobservableangularfire

Answer №1

Your current code seems to have some issues and may not be the most efficient way of handling data. Instead of downloading all data and then filtering on the client side, it would be better to filter the data directly on the server side. This way, you can avoid downloading unnecessary information that your users won't even see.

A helpful approach is demonstrated in the AngularFire2 documentation:

return this.afDb.list('articles', {
  query: {
    orderByChild: 'validUntil',
    startAt: '2017-09-29T21:00:00.000Z' 
  }
});

Answer №2

To enhance the filtering process for articles, utilize a combination of map and filter:

getValidArticles(): Observable<Article[]> {
  return this.afDb.list('articles')
    .map((articles) =>
      articles.filter((article) =>
        new Date(article.validUntil).getTime() >= new Date().getTime()));
}

If you are seeing two console.log outputs, it could be due to subscribing twice to the observable.

If server-side filtering is not supported by the AngularFire framework, consider following Frank van Puffelen's recommendation.

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