How to modify the BatchMapping Source Object after an operation

[ShreeGiriKumar]

I am trying to modify the BatchMapping source object right before sending back the response. Imagine there is some db/api exception while fetching ratings for a specific movie and I would like to send that exception info part of the Movie object. Below is a sample snippet and I would like to know if its achievable.

@QueryMapping(name = "getMovies")
public List getAllMovies() {
return Movie.getMovies();
}

@BatchMapping(typeName = "Movie", field = "ratings")
public Map<Movie, List<Rating>> rating(List<Movie> movies) {
    // Use the context to get the batch loader

    Map<Movie, List<Rating>> movieRatings = movies.stream()
            .collect(Collectors.toMap(movie -> movie, movie -> Rating.getById(movie.getId())));
    return movieRatings;
}

private Movie modify(Movie movie){
    if(movie.getId().equalsIgnoreCase("movie-1")) { //simulating an exception
        movie.setError("error while fetching ratings");
        return movie;
    }
    return movie;
}

[bclozel]

I don't see this modify method being called in this code snippet. Maybe I am missing something?

Also could you explain why the data loader isn't throwing an error in the first place?

[ShreeGiriKumar]

@BatchMapping(typeName = "Movie", field = "ratings")
public Map<Movie, List<Rating>> rating(List<Movie> movies) {
    // Use the context to get the batch loader

    Map<Movie, List<Rating>> movieRatings = movies.stream()
            .collect(Collectors.toMap(movie -> modify(movie), movie -> Rating.getById(movie.getId())));
    return movieRatings;
}
private Movie modify(Movie movie){
    if(movie.getId().equalsIgnoreCase("movie-1")) { //simulating an exception
        movie.setError("error while fetching ratings");
        return movie;
    }
    return movie;
}

So, we have the exceptions handled in the service layer. So, the expected response be like,

id: 1
name: "abc"
error: null

id: 0
name: null
error: {
    code: 500
    message: "exception fetching ratings"
}