javascript - Combine multiple observable arrays into new object array

Question

I have 3 observable arrays like below.

persons = [
   {
      "firstName":"john",
      "lastName":"public",
      "locationID":"1",
      "departmentID":"100"
   },
   {
      "firstName":"sam",
      "lastName":"smith",
      "locationID":"2",
      "departmentID":"101"
   }
]

departments = [{"departmentID": "100",
               "name": "development"
               },
               {"departmentID": "101",
                "name": "sales"
               }]

locations = [{"locationID": "1", "name": "chicago"},
              {"locationID":"2", "name": "ny"}]

I am trying to combine these 3 into below result ,

result = [
   {
      "firstName":"john",
      "lastName":"public",
      "location":"development",
      "department":"sales"
   },
   {
      "firstName":"sam",
      "lastName":"smith",
      "location":"ny",
      "department":"sales"
   }
]

To get the desired result, I have used map function on persons observable to give new object array.

this.store<Person>('persons')
.map(function(person){
     let p = new personDetail()
     p.firstName = person.firstName,
     p.lastName = person.lastName
     return p;
})

PersonDetail object has firstName, lastName, location and department properties. How do I do a lookup into departments observable and get a matching row for departmentID to get the department name ?

I am new to rxjs library, please let me know if there is a better way to attain the desired result.

Answer 1

Since you'll very likely want to fetch lists of departments and locations from a remote service (make another HTTP request) I'd do it right away with Observables as well.

Observable.from(persons)
    .mergeMap(person => {
        let department$ = Observable.from(departments)
            .filter(department => department.departmentID == person.departmentID);

        let location$ = Observable.from(locations)
            .filter(location => location.locationID == person.locationID);

        return Observable.forkJoin(department$, location$, (department, location) => {
            return {
                'firstName': person.firstName,
                'lastName': person.lastName,
                'location': location.name,
                'department': department.name,
            };
        });
    })
    .toArray()
    .subscribe(result => console.log(result));

This prints to console:

[ { firstName: 'john',
    lastName: 'public',
    location: 'chicago',
    department: 'development' },
  { firstName: 'sam',
    lastName: 'smith',
    location: 'ny',
    department: 'sales' } ]

There're two Observables department$ and location$ that are filtered with filter() operator to get the only item with matching ID. Then forkJoin() operator waits until both of them are complete. Operator mergeMap() then reemits the value returned from forkJoin(). At the end with toArray() we collect all items into a single array.

Instead of Observable.from(...) you can have whatever service you'll need (eg. http.get(...)).

See live demo: https://jsbin.com/nenekup/4/edit?js,console

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