fix(storage): return Mongo List keys in key order like the memory store

internal/storage/doc_store.go

@@ -40,8 +40,9 @@ var ErrInvalidModuleName = errors.New("storage: invalid module name")
 //   - PutVersioned writes only if the stored version equals expectedVersion,
 //     then bumps it. expectedVersion == 0 means "create (or adopt a not-yet-
 //     versioned key)". A mismatch returns ErrConflict.
-//   - List returns the keys under a prefix; Scan returns those keys with their
-//     values, ordered by key ascending.
+//   - List returns the keys under a prefix, ordered by key ascending.
+//   - Scan returns the keys under a prefix with their values, ordered by key
+//     ascending.
 type DocStore[T any] interface {
 	Get(ctx context.Context, id string) (val T, version int64, err error)
 	Put(ctx context.Context, id string, val T) error

internal/storage/mongo_doc_store.go

@@ -153,7 +153,8 @@ func (s *mongoDocStore[T]) Delete(ctx context.Context, id string) error {
 
 // List returns all document IDs whose key starts with prefix, via a half-open
 // range scan on _id (reusing prefixSuccessor) so it uses the _id index and
-// avoids regex injection. Empty prefix returns the whole collection.
+// avoids regex injection. Keys come back sorted by _id, matching the memory
+// store. Empty prefix returns the whole collection.
 func (s *mongoDocStore[T]) List(ctx context.Context, prefix string) ([]string, error) {
 	if err := validatePrefix(prefix); err != nil {
 		return nil, err
@@ -162,7 +163,9 @@ func (s *mongoDocStore[T]) List(ctx context.Context, prefix string) ([]string, e
 	if prefix != "" {
 		filter[mongoIDField] = bson.M{"$gte": prefix, "$lt": prefixSuccessor(prefix)}
 	}
-	cur, err := s.coll.Find(ctx, filter, options.Find().SetProjection(bson.M{mongoIDField: 1}))
+	cur, err := s.coll.Find(ctx, filter, options.Find().
+		SetProjection(bson.M{mongoIDField: 1}).
+		SetSort(bson.D{{Key: mongoIDField, Value: 1}}))
 	if err != nil {
 		return nil, fmt.Errorf("mongo list %s prefix=%q: %w", s.module, prefix, err)
 	}

internal/storage/mongo_doc_store_test.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"os"
+	"slices"
 	"sync"
 	"testing"
 	"time"
@@ -235,3 +236,25 @@ func TestMongoDocStore_ScanReturnsPayloadsInKeyOrder(t *testing.T) {
 		t.Fatalf("Scan all = %d docs, err %v", len(all), err)
 	}
 }
+
+// List returns keys in key order, like the memory store, so callers behave the
+// same on both backends. Insertion order is deliberately reversed here.
+func TestMongoDocStore_ListReturnsKeysInKeyOrder(t *testing.T) {
+	store, _, cleanup := mongoStore[portfolioLike](t, "coin")
+	defer cleanup()
+	ctx := context.Background()
+
+	for _, key := range []string{"user:3", "user:1", "other", "user:2"} {
+		if err := store.Put(ctx, key, portfolioLike{}); err != nil {
+			t.Fatalf("Put %s: %v", key, err)
+		}
+	}
+
+	keys, err := store.List(ctx, "user:")
+	if err != nil {
+		t.Fatalf("List: %v", err)
+	}
+	if want := []string{"user:1", "user:2", "user:3"}; !slices.Equal(keys, want) {
+		t.Errorf("List = %v, want %v", keys, want)
+	}
+}