081_Fetching_Query_Specificationsx20

Fetching Query Specifications

Wolverine recognizes Marten query specifications — either compiled queries (ICompiledQuery<TDoc, TResult>) or query plans (IQueryPlan<T> / IBatchQueryPlan<T>) — when they appear as handler dependencies, executes them automatically, and batches them into a single Marten IBatchedQuery whenever possible, including alongside [Entity] and [Aggregate] loads on the same handler.

This gives you two ergonomics for the same underlying machinery:

1. Return the specification directly from a Load() method. Wolverine sees the return type and wires up execution + relay to Handle / Validate parameters.
2. Annotate a handler parameter with [FromQuerySpecification]. Wolverine constructs the spec from the message (and other variables in scope) and runs it.

Under the hood both paths produce the same codegen — specifications are collected and run through a single CreateBatchQuery() whenever two or more can be batched together.

Returning a specification from Load

The handler’s Load (or LoadAsync) method returns the specification object. Wolverine recognizes the return type and executes it; the materialized result is what the subsequent Handle / Validate method receives.

public class NoteByIdCompiled : ICompiledQuery<Note, Note?>
{
    public Guid NoteId { get; set; }
 
    public Expression<Func<IMartenQueryable<Note>, Note?>> QueryIs()
        => q => q.FirstOrDefault(x => x.Id == NoteId);
}
 
public class LoadOneNoteHandler
{
    // The return type IS the specification. Wolverine runs it and passes
    // the materialized Note? to Handle.
    public static NoteByIdCompiled Load(LoadOneNote cmd)
        => new() { NoteId = cmd.NoteId };
 
    public static void Handle(LoadOneNote cmd, Note? note)
    {
        // note is the materialized query result
    }
}

Tuple returns — multiple specifications in one round-trip

When a Load returns a ValueTuple of specifications, Wolverine treats each tuple element as its own specification and batches them:

public class LoadNoteAndListStarredHandler
{
    public static (NoteByIdCompiled, StarredNotesPlan) Load(LoadNoteAndListStarred cmd)
        => (new() { NoteId = cmd.NoteId }, new());
 
    public static void Handle(LoadNoteAndListStarred cmd, Note? note, IReadOnlyList<Note> notes)
    {
        // Both the Note and the list arrive in one IBatchedQuery round-trip
    }
}

The generated code is roughly:

var (spec1, spec2) = Handler.Load(cmd);
var batch = documentSession.CreateBatchQuery();
var noteTask  = batch.Query<Note, Note?>(spec1);
var notesTask = batch.QueryByPlan<IReadOnlyList<Note>>((IBatchQueryPlan<IReadOnlyList<Note>>)spec2);
await batch.Execute(cancellation);
var note  = await noteTask;
var notes = await notesTask;
Handler.Handle(cmd, note, notes);

[FromQuerySpecification] — spec construction driven by the attribute

When you don’t need a dedicated Load method — the specification’s inputs are all available on the message itself — attach [FromQuerySpecification(typeof(TSpec))] to the handler parameter:

public class LoadNoteViaAttributeHandler
{
    public static void Handle(
        LoadNoteViaAttribute cmd,
        [FromQuerySpecification(typeof(NoteByIdCompiled))] Note? note)
    {
        // Wolverine constructs new NoteByIdCompiled(), sets NoteId from
        // cmd.NoteId by name match, runs it, and passes the result here.
    }
}

Wolverine:

1. Picks the spec’s public constructor with the most parameters
2. Resolves each constructor parameter from variables in scope (message members, route values, headers, claims) by case-insensitive name match
3. Assigns any remaining public settable properties on the spec from variables in scope — the canonical Marten compiled-query pattern where parameters are properties

On .NET 7+ you can use the generic variant for less ceremony:

public static void Handle(
    LoadNoteViaAttribute cmd,
    [FromQuerySpecification<NoteByIdCompiled>] Note? note)
{
}

Batching with [Entity], [Aggregate], and other specs

Specifications, [Entity] loads, and [Aggregate] loads all share the same batching machinery. A handler that mixes them loads everything in one IBatchedQuery round-trip:

public class ApproveOrderHandler
{
    public static (NoteByIdCompiled, StarredNotesPlan) Load(ApproveOrder cmd)
        => (new() { NoteId = cmd.NoteId }, new());
 
    public static void Handle(
        ApproveOrder cmd,
        Note? note,                         // from compiled query
        IReadOnlyList<Note> starred,        // from query plan
        [Entity] Customer customer,         // from [Entity] by id
        [Aggregate] Order order)            // from [Aggregate] event stream
    {
        // All four loads executed in a single IBatchedQuery round-trip.
    }
}

Which specifications can be batched?

Specification kind	Batched?
ICompiledQuery<TDoc, TResult>	Always — via IBatchedQuery.Query(compiled)
A class that implements IBatchQueryPlan<T> (including anything deriving from QueryListPlan<T>)	Always — via IBatchedQuery.QueryByPlan(plan)
A class that implements only IQueryPlan<T> (no batch variant)	Standalone — runs via session.QueryByPlanAsync(plan) after the batch finishes

When a plan cannot be batched, it executes as a separate round-trip; the rest of the specifications on the same handler still share one batch.

Tip: if you’re writing a plan that returns a list, prefer inheriting from QueryListPlan<T> — it implements both IQueryPlan<IReadOnlyList<T>> and IBatchQueryPlan<IReadOnlyList<T>>, so your plan is batch-capable for free.

When to reach for each approach

Situation	Recommendation
Load a single entity by primary key	[Entity]
Load an event-sourced aggregate	[Aggregate] / [ReadAggregate] / [WriteAggregate]
Reusable predicate used in multiple handlers	Extract to an IQueryPlan<T> and return from Load (or use [FromQuerySpecification])
Hot, repeatedly-executed LINQ query	Extract to ICompiledQuery<,> and return from Load (or use [FromQuerySpecification])
One-off LINQ that isn’t reused elsewhere	Inline in the handler body

Refactor heuristic: when you find the same session.Query<T>().Where(...) expression in two or more handlers, extract it to a plan or compiled query. Return it from Load. Wolverine will handle the rest — including batching it with other loads on the same handler for a free performance win.

Cross-provider parity

The same ergonomic applies to Wolverine.EntityFrameworkCore — Load methods returning EF Core IQueryPlan<TDbContext, TResult> instances (or tuples of them) auto-execute, and [FromQuerySpecification] works there too. See Query Plans (EF Core) for the parallel story.

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url: /guide/messaging/transports/sqs/fifo-queues.md