RxJS: Combining two http.get() to one object - json

Question

I'm using WP Rest API to access my Wordpress database with Angular 2. I use their post API and return a json object which I map to create an object.
Object:
export class BlogPost {
date: Date;
id: number;
link: string;
title: string;
author: number;
excerpt: string;
featuredMediaURL: string;
featuredMediaID: number;
categories: string[];
tags: string[];
constructor(obj?: any) {
this.date = obj && obj.date || null;
this.id = obj && obj.id || null;
this.link = obj && obj.link || null;
this.title = obj && obj.title || null;
this.author = obj && obj.author || null;
this.excerpt = obj && obj.excerpt || null;
this.featuredMediaURL = obj && obj.featuredMediaURL || null;
this.featuredMediaID = obj && obj.featuredMediaID || null;
this.categories = obj && obj.categories || null;
this.tags = obj && obj.tags || null;
}
}
Service:
export class BlogService {
constructor(public http: Http,
#Inject(DOMAIN) private domain: string) { }
getPost(): Observable<BlogPost[]> {
return this.http.get(`${this.domain}/wp-json/wp/v2/posts/?page=1`)
.map((response: Response) => {
return (<any>response.json()).map(item => {
return new BlogPost({
date: item.date,
id: item.id,
link: item.link,
title: item.title,
author: item.author,
excerpt: item.excerpt,
featuredMediaID: item.featured_media,
categories: item.categories,
tags: item.tags
});
});
});
}
My problem is that for certain properties such as media, the API returns an ID for the media and not the url. I was able to mix and match and get the url into the template by creating a separate service which returns all media stored on the database and then compares the id numbers between the two arrays and returns the correct one:
mediaURL(id: number): string {
for (let med of this.media) {
if(med.id === id) {
return med.url;
};
};
return 'error';
}
but this is a bit tedious. I was wondering if there is a clean rxjs way to do this by issuing a
this.http.get(`${this._domain}/wp-json/wp/v2/media/<id>`)
line and mapping it to the BlogPost.featuredMediaURL before the whole array of posts is returned to the blog component.
I've messed around with it for a day trying several things and I'm at a loss.