Updates following code review.

Author: ajcvickers

src/MongoDB.Driver/CreateVectorSearchIndexModelBase.cs

@@ -16,6 +16,7 @@
 using System.Collections.Generic;
 using System.Linq;
 using MongoDB.Bson;
+using MongoDB.Driver.Core.Misc;
 
 namespace MongoDB.Driver;
 
@@ -86,6 +87,8 @@ protected CreateVectorSearchIndexModelBase(
         params FieldDefinition<TDocument>[] filterFields)
         : base(name, SearchIndexType.VectorSearch)
     {
+        Ensure.IsNotNull(field, nameof(field));
+
         Field = field;
         FilterFields = filterFields?.ToList() ?? [];
     }
@@ -152,11 +155,12 @@ private protected void RenderCommonElements(RenderArgs<TDocument> renderArgs, Bs
     /// <param name="fieldDocument">The field document into which the elements will go.</param>
     private protected void RenderCommonFieldElements(RenderArgs<TDocument> renderArgs, BsonDocument fieldDocument)
     {
-        var quantizationValue = Quantization == VectorQuantization.BinaryNoRescore
-            ? "binaryNoRescore"
-            : Quantization?.ToString().ToLowerInvariant();
-
-        fieldDocument.Add("quantization", quantizationValue, quantizationValue != null);
+        if (Quantization != null && Quantization != VectorQuantization.None)
+        {
+            fieldDocument.Add("quantization", Quantization == VectorQuantization.BinaryNoRescore
+                ? "binaryNoRescore"
+                : Quantization.ToString().ToLowerInvariant());
+        }
 
         if (HnswMaxEdges != null || HnswNumEdgeCandidates != null)
         {

src/MongoDB.Driver/PipelineStageDefinitionBuilder.cs

@@ -2183,8 +2183,14 @@ public static PipelineStageDefinition<TInput, TInput> VectorSearch<TInput>(
                         { "returnStoredSource", true, options?.ReturnStoredSource == true },
                     };
 
-                    if (nestedRoot != null && options?.NestedFilter != null)
+                    if (options?.NestedFilter != null)
                     {
+                        if (nestedRoot == null)
+                        {
+                            throw new InvalidOperationException(
+                                $"A nested filter was specified for the search against field '{path}', but the field is not nested. Nested filters can only be used when searching against vectors in nested (embedded) documents.");
+                        }
+
                         vectorSearchOperator.Add("filter", options.NestedFilter.Render(
                             (args with { SubPathRoot = nestedRoot }) with { RenderDollarForm = true }));
                     }

tests/MongoDB.Driver.Tests/PipelineDefinitionBuilderTests.cs

@@ -707,6 +707,7 @@ private class MovieWithPlot
         {
             public int Year { get; set; }
             public NestedPlot Plot { get; set; }
+            public float[] NonNestedEmbedding  { get; set; }
         }
 
         private class NestedPlot
@@ -738,6 +739,28 @@ public void VectorSearch_should_throw_when_filter_is_at_wrong_level([Values(fals
                 .Which.Message.Should().Contain("The field 'Year' is not part of the nested document 'Plot', which is the root of this search.");
         }
 
+        [Theory]
+        [ParameterAttributeData]
+        public void VectorSearch_should_throw_when_nested_filter_is_used_without_nesting([Values(false, true)] bool expressions)
+        {
+            var pipeline = new EmptyPipelineDefinition<MovieWithPlot>();
+            var options = new VectorSearchOptions<MovieWithPlot>()
+            {
+                IndexName = "index_name",
+            };
+
+            options.NestedFilter = expressions
+                ? Builders<MovieWithPlot>.Filter.Lt(m => m.Plot.Rating, 1900)
+                : Builders<MovieWithPlot>.Filter.Lt("Plot.Rating", 1900);
+
+            var result = pipeline.VectorSearch(m => m.NonNestedEmbedding, new[] { 1.0, 2.0, 3.0 }, 1, options);
+
+            var exception = Record.Exception(() => RenderStages(result, BsonSerializer.SerializerRegistry.GetSerializer<MovieWithPlot>()));
+
+            exception.Should().BeOfType<InvalidOperationException>()
+                .Which.Message.Should().Contain("A nested filter was specified for the search against field 'NonNestedEmbedding',");
+        }
+
         [Fact]
         public void VectorSearch_should_throw_when_pipeline_is_null()
         {