Fix workflow list MongoDB fallback

pkg/workflow/db/mongo.go

@@ -47,13 +47,13 @@ func ListMongo(ctx context.Context, db *mongo.Database, collection string, opts
 	regex := fmt.Sprintf("^%s.*%s$", prefix, suffix)
 
 	filter := bson.M{
-		"key": bson.M{
+		"_id": bson.M{
 			"$regex":   regex,
 			"$options": "",
 		},
 	}
 
-	findOpts := options.Find().SetProjection(bson.M{"_id": 0, "key": 1})
+	findOpts := options.Find().SetProjection(bson.M{"_id": 1})
 
 	cur, err := coll.Find(ctx, filter, findOpts)
 	if err != nil {
@@ -64,7 +64,7 @@ func ListMongo(ctx context.Context, db *mongo.Database, collection string, opts
 	var keys []string
 	for cur.Next(ctx) {
 		var doc struct {
-			Key string `bson:"key"`
+			Key string `bson:"_id"`
 		}
 		if err := cur.Decode(&doc); err != nil {
 			return nil, err

pkg/workflow/db/mongo_test.go

@@ -0,0 +1,86 @@
+/*
+Copyright 2026 The Dapr Authors
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+    http://www.apache.org/licenses/LICENSE-2.0
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package db
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.mongodb.org/mongo-driver/bson"
+	"go.mongodb.org/mongo-driver/mongo/integration/mtest"
+)
+
+func TestListMongo(t *testing.T) {
+	mt := mtest.New(t, mtest.NewOptions().ClientType(mtest.Mock))
+
+	mt.Run("returns keys from _id field", func(mt *mtest.T) {
+		key1 := "myapp||dapr.internal.default.myapp.workflow||instance-1||metadata"
+		key2 := "myapp||dapr.internal.default.myapp.workflow||instance-2||metadata"
+
+		mt.AddMockResponses(mtest.CreateCursorResponse(1, "daprStore.daprCollection", mtest.FirstBatch,
+			bson.D{{Key: "_id", Value: key1}},
+		), mtest.CreateCursorResponse(0, "daprStore.daprCollection", mtest.NextBatch,
+			bson.D{{Key: "_id", Value: key2}},
+		))
+
+		keys, err := ListMongo(mt.Context(), mt.DB, "daprCollection", ListOptions{
+			Namespace: "default",
+			AppID:     "myapp",
+		})
+		require.NoError(mt, err)
+		assert.Equal(mt, []string{key1, key2}, keys)
+	})
+
+	mt.Run("returns nil when no documents match", func(mt *mtest.T) {
+		mt.AddMockResponses(mtest.CreateCursorResponse(0, "daprStore.daprCollection", mtest.FirstBatch))
+
+		keys, err := ListMongo(mt.Context(), mt.DB, "daprCollection", ListOptions{
+			Namespace: "default",
+			AppID:     "myapp",
+		})
+		require.NoError(mt, err)
+		assert.Nil(mt, keys)
+	})
+
+	mt.Run("filter uses _id field with correct regex", func(mt *mtest.T) {
+		mt.AddMockResponses(mtest.CreateCursorResponse(0, "daprStore.daprCollection", mtest.FirstBatch))
+
+		_, err := ListMongo(mt.Context(), mt.DB, "daprCollection", ListOptions{
+			Namespace: "default",
+			AppID:     "myapp",
+		})
+		require.NoError(mt, err)
+
+		// Verify the find command was sent with _id filter.
+		cmd := mt.GetStartedEvent().Command
+		filterVal := cmd.Lookup("filter")
+		filterDoc := filterVal.Document()
+
+		// The filter should contain an "_id" field, not "key".
+		idVal, err := filterDoc.LookupErr("_id")
+		assert.NoError(mt, err, "filter should query on _id field")
+		_, err = filterDoc.LookupErr("key")
+		assert.Error(mt, err, "filter should not query on key field")
+
+		// Verify the $regex value matches the expected workflow metadata pattern.
+		idDoc := idVal.Document()
+		regexVal, err := idDoc.LookupErr("$regex")
+		require.NoError(mt, err, "filter on _id should use $regex")
+		assert.Equal(mt,
+			`^myapp\|\|dapr\.internal\.default\.myapp\.workflow\|\|.*\|\|metadata$`,
+			regexVal.StringValue(),
+		)
+	})
+}