Cross-Schema Data Loaders

EF Core cannot JOIN across two DbContext instances, so a GraphQL field that resolves an entity owned by a different schema/context has no SQL join available and would otherwise run one query per parent row (N+1). CrossSchemaLoader<TContext, TEntity> (in Trax.Api.GraphQL) batches those lookups: every key requested in a GraphQL request collapses into a single WHERE id IN (...) against the owning context.

CrossSchemaLoader

A GreenDonut.BatchDataLoader<int, TEntity> keyed by integer Id. It injects the pooled IDbContextFactory<TContext> and creates a short-lived context per batch (a DbContext is not thread-safe). You do not subclass it; you reference the closed type in an edge resolver.

[ExtendObjectType(typeof(Loan))]
public sealed class LoanToBookEdge
{
    public async Task<Book?> GetBook(
        [Parent(requires: nameof(Loan.BookId))] Loan loan,
        CrossSchemaLoader<CatalogDbContext, Book> books,
        CancellationToken ct
    ) => await books.LoadAsync(loan.BookId, ct);
}

The target entity must have an integer Id primary key.

requires: is what puts the foreign key in the projection. Projection narrows the SELECT to the columns the caller named, and a cross-schema edge reads a column the caller did not ask for, so without the declaration loan.BookId arrives as 0 and the edge resolves to null. Trax adds the key of a query model to the projection on its own; a foreign key has to be declared. See query models.

The ExtensionResolversDeclareParentRequirements guard fails the build when an edge resolver omits it.

CrossSchemaEdge and registration

Declare every edge in one manifest (the single source of truth the architecture guards verify), and register one loader per distinct target pair:

public static readonly IReadOnlyList<CrossSchemaEdge> All =
[
    new(Source: typeof(Loan), Fk: nameof(Loan.BookId), Target: typeof(Book),
        TargetContext: typeof(CatalogDbContext), FieldName: "book"),
];
 
// In your service registration:
services.AddCrossSchemaLoader<CatalogDbContext, Book>();

Cross-schema edge resolvers live in their own project (the one place allowed to reference more than one domain context), keeping the domains isolated from each other.

SDK Reference

Query models | Architecture guards