How to create Flux from a Stream that returns Mono?

Imagine I have a list of coursename objects with (CourseID, Name). Let's call that list as 'courseNameList'.

When someone send's request to "<IP:port>/courses" we want to display list of courses.

However, before sending the results, I also need to append price of each course. The price will be retrieved from another Microservice and it returns a Mono object.

So, user will see list of courses with (ID, Name, Price). Where price comes from another service.

The controller method might look like this

@GetMapping("/courses")
public Flux<Course> gerProducts() {
 courseNameList.stream.map(courseName -> {
      //Make a webClient call to pricing service by sending coureName.getID as parameter and get the 'price' Mono object
      //return the course object with id, name, price
   })
     //Collect and return Flux with contains list of courses!
}

I tried multiple ways to return the Flux<Course>. But, I am unable to figure out how. I need those comments to be replaced with equivalent (Or better) code.

Starting with a flux rather than a list would be better, but if not possible then create the flux from a stream as below and then use flatMap instead of map.

@GetMapping("/courses")
public Flux<Course> gerProducts() {
  return Flux.fromStream(courseNameList.stream()).flatMap(courseName -> {
    // Make webClient call which returns mono
  });
}

You can take a look at this example and come up with a solution yourself.

// I assumed you have courses list already as you are using a list.
 List<String> list = List.of("a", "bb", "ccccc", "ddd");
 Flux.fromIterable(list)
       // this is where you find the price and append
       .flatMap(a -> Mono.just(a).map(k -> k + ":" + k.length()))                 
       .subscribe(System.out::println);

//output
a:1
bb:2
ccccc:5
ddd:3

But if you have 100 courses, will you want to make 100 calls to the service one by one. Will it not affect the performance? Instead can you send the list of courses and get the price from the service in 1 call?

// you have courses list
List<String> list = List.of("a", "bb", "ccccc", "ddd");

// get the price of all courses in 1 call
Mono.fromSupplier(() -> List.of(1, 2, 5, 3))
        .flatMapIterable(Function.identity())
        .index()
        // map the courses and price
        // I used index. but you could use courseID to map the price
        .map(t -> list.get(t.getT1().intValue()) + ":" + t.getT2())
        .subscribe(System.out::println);