DataLoaders

The rule

A field on a list item never sends its own query. Asking for the reviews of 50 movies must cost one Application query, not 50. DataLoaders in src/Presentation/GraphQL/DataLoaders/ collect the ids asked for during one request and load them together.

They're static methods with [DataLoader]. The source generator creates an interface and class for each one: GetReviewsByMovieIdAsync becomes IReviewsByMovieIdDataLoader. Each takes the ids, then ISender and the token, and returns a Dictionary<id, value> with an entry for every id, empty when there's nothing.

Batches are split into chunks of Paging.MaxLimit, because the Application queries refuse more ids than that.

Steps

1. Add an Application query that takes a list of ids and returns a dictionary (add-a-use-case), like GetReviewsForMoviesQuery.
2. Add a [DataLoader] method that chunks the ids by Paging.MaxLimit and sends the query for each chunk.
3. Return an entry for every id asked for, so callers can use LoadRequiredAsync.
4. In the type, add a resolver that takes [Parent], the generated DataLoader interface and the token, and calls LoadRequiredAsync(parent.Id, cancellationToken).
5. Test a list query that asks for the child field, and check the counts match (presentation-integration-tests).

Example

// src/Presentation/GraphQL/Types/MovieSummaryType.cs
public static async Task<List<Review>> GetReviewsAsync(
    [Parent] MovieSummary movie,
    IReviewsByMovieIdDataLoader reviews,
    CancellationToken cancellationToken)
{
    return await reviews.LoadRequiredAsync(movie.Id, cancellationToken);
}

Don't

• Call sender.Send from a resolver on a list item.
• Return a dictionary missing some ids.
• Send more than Paging.MaxLimit ids in one Application query.

Check

dotnet test tests/Presentation.Tests.Integration --filter "FullyQualifiedName~Movies|FullyQualifiedName~Authors"