Fix vector order lookup with composite ordering

LiteDB.Tests/Query/VectorIndex_Tests.cs

@@ -264,6 +264,52 @@ public void TopKNear_UsesVectorIndex()
             results.Select(x => x.Id).Should().Equal(new[] { 1, 3 });
         }
 
+        [Fact]
+        public void OrderBy_VectorSimilarity_WithCompositeOrdering_UsesVectorIndex()
+        {
+            using var db = new LiteDatabase(":memory:");
+            var collection = db.GetCollection<VectorDocument>("vectors");
+
+            collection.Insert(new[]
+            {
+                new VectorDocument { Id = 1, Embedding = new[] { 1f, 0f }, Flag = true },
+                new VectorDocument { Id = 2, Embedding = new[] { 1f, 0f }, Flag = false },
+                new VectorDocument { Id = 3, Embedding = new[] { 0f, 1f }, Flag = true }
+            });
+
+            collection.EnsureIndex(
+                "embedding_idx",
+                BsonExpression.Create("$.Embedding"),
+                new VectorIndexOptions(2, VectorDistanceMetric.Cosine));
+
+            var similarity = BsonExpression.Create("VECTOR_SIM($.Embedding, [1.0, 0.0])");
+
+            var query = (LiteQueryable<VectorDocument>)collection.Query()
+                .OrderBy(similarity, Query.Ascending)
+                .ThenBy(x => x.Flag);
+
+            var queryField = typeof(LiteQueryable<VectorDocument>).GetField("_query", BindingFlags.NonPublic | BindingFlags.Instance);
+            var definition = (Query)queryField.GetValue(query);
+
+            definition.OrderBy.Should().HaveCount(2);
+            definition.OrderBy[0].Expression.Type.Should().Be(BsonExpressionType.VectorSim);
+
+            definition.VectorField = "$.Embedding";
+            definition.VectorTarget = new[] { 1f, 0f };
+            definition.VectorMaxDistance = double.MaxValue;
+
+            var plan = query.GetPlan();
+
+            plan["index"]["mode"].AsString.Should().Be("VECTOR INDEX SEARCH");
+            plan["index"]["expr"].AsString.Should().Be("$.Embedding");
+            plan.ContainsKey("orderBy").Should().BeFalse();
+
+            var results = query.ToArray();
+
+            results.Should().HaveCount(3);
+            results.Select(x => x.Id).Should().BeEquivalentTo(new[] { 1, 2, 3 });
+        }
+
         [Fact]
         public void WhereNear_DotProductHonorsMinimumSimilarity()
         {

LiteDB/Engine/Query/QueryOptimization.cs

@@ -326,12 +326,16 @@ private bool TrySelectVectorIndex(out VectorIndexQuery index, out BsonExpression
                 }
             }
 
-            if (expression == null && _query.OrderBy != null)
+            if (expression == null && _query.OrderBy.Count > 0)
             {
-                if (this.TryParseVectorExpression(_query.OrderBy, out expression, out target))
+                foreach (var order in _query.OrderBy)
                 {
-                    matchedFromOrderBy = true;
-                    maxDistance = double.MaxValue;
+                    if (this.TryParseVectorExpression(order.Expression, out expression, out target))
+                    {
+                        matchedFromOrderBy = true;
+                        maxDistance = double.MaxValue;
+                        break;
+                    }
                 }
             }
 
@@ -340,7 +344,7 @@ private bool TrySelectVectorIndex(out VectorIndexQuery index, out BsonExpression
                 expression = NormalizeVectorField(_query.VectorField);
                 target = _query.VectorTarget?.ToArray();
                 maxDistance = _query.VectorMaxDistance;
-                matchedFromOrderBy = matchedFromOrderBy || (_query.OrderBy != null && _query.OrderBy.Type == BsonExpressionType.VectorSim);
+                matchedFromOrderBy = matchedFromOrderBy || (_query.OrderBy.Any(order => order.Expression?.Type == BsonExpressionType.VectorSim));
             }
 
             if (expression == null || target == null)