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"
rating : [{
id: 1,
rating: 2
]
error: null

id: 2
name: "def"
rating: null,
error: {
    code: 500
    message: "exception fetching ratings"
}

[bclozel]

I don't think you can nor should modify the source for a batch operation. @BatchMapping operations are shortcut for registering a batch loader operation. Such operations can be called once or multiple times during the execution of the GraphQL request. They can be called from different parts of the request in a batched fashion. Also, results are cached for the duration of the request.

In your case, if the same movie is fetched multiple times within a single request, only the first Movie instance might be updated. I think you could consider returning Try<Rating> from your batch loading function and let the engine handle errors you would return for failures.

I understand that it's a well-known design choice to return both errors and data in the same type, but in those cases you should do so directly on the Rating type, not its parent.

The GraphQL Java team has most likely a deeper understanding of this, maybe you can create a new discussion there? I don't think you should point to the Spring for GraphQL case you used in this issue, but more generally ask if and how you should report errors in the parent type when an issue happens while batch loading values.

I'll close this issue for now but we can reopen it if we find out we can improve support in this project. Thanks!