DataLoader

DataLoaders are a foundational concept in GraphQL and allow us to coalesce multiple requests for the same entity into a single request. This is especially useful when we have a list of entities and need to fetch related entities for each of them. Without a DataLoader, we would have to make a separate request for each entity, which can lead to the N+1 problem.

DataLoader in Hot Chocolate

In Hot Chocolate, we can implement a class-based DataLoader by inheriting from the DataLoaderBase<TKey, TValue> class. It also offers more specialized DataLoader base classes like the BatchDataLoader<TKey, TValue>.

public sealed class ProductByIdDataLoader : BatchDataLoader<int, ProductInfo>, IProductByIdDataLoader
{
    public ProductByIdDataLoader(
        Func<HttpClient> clientFactory,
        IBatchScheduler batchScheduler,
        DataLoaderOptions options)
        : base(batchScheduler, options)
    {
    }
}

But Hot Chocolate also provides a DataLoader to remove boilerplate code from less complex DataLoader implementations:

internal static class ProductDataLoader
{
    [DataLoader]
    public static async Task<Dictionary<int, Product>> GetProductByIdAsync(
        IReadOnlyList<int> ids,
        CatalogContext context,
        CancellationToken ct)
        => await context.Products
            .AsNoTracking()
            .Where(t => ids.Contains(t.Id))
            .ToDictionaryAsync(t => t.Id, ct);
}

In the above example, the DataLoader is basically reduced to its fetch method, and the class with DI logic is generated by the source generator.