add simple tests

Author: adelinowona

src/MongoDB.Driver/SortDefinitionBuilder.cs

@@ -250,6 +250,11 @@ internal sealed class CombinedSortDefinition<TDocument> : SortDefinition<TDocume
         public CombinedSortDefinition(IEnumerable<SortDefinition<TDocument>> sorts)
         {
             _sorts = Ensure.IsNotNull(sorts, nameof(sorts)).ToList();
+
+            if (_sorts.Any(sort => sort is NoFieldDirectionalSortDefinition<TDocument>))
+            {
+                throw new InvalidOperationException("Value-based sort cannot be combined with other sorts. When sorting by the entire element value, no other sorting criteria can be applied.");
+            }
         }
 
         public override BsonDocument Render(RenderArgs<TDocument> args)

tests/MongoDB.Driver.Tests/SortDefinitionBuilderTests.cs

@@ -13,6 +13,7 @@
 * limitations under the License.
 */
 
+using System;
 using FluentAssertions;
 using MongoDB.Bson;
 using MongoDB.Bson.Serialization;
@@ -31,6 +32,14 @@ public void Ascending()
             Assert(subject.Ascending("a"), "{a: 1}");
         }
 
+        [Fact]
+        public void Ascending_no_field()
+        {
+            var subject = CreateSubject<BsonDocument>();
+
+            Assert(subject.Ascending(), "{direction: 1}");
+        }
+
         [Fact]
         public void Ascending_Typed()
         {
@@ -76,6 +85,16 @@ public void Combine_with_repeated_fields_using_extension_methods()
             Assert(sort, "{b: -1, a: -1}");
         }
 
+        [Fact]
+        public void Combine_with_value_based_sort_and_additional_sort_should_throw()
+        {
+            var subject = CreateSubject<BsonDocument>();
+
+            var exception = Record.Exception(() => subject.Ascending().Descending("b"));
+
+            exception.Should().BeOfType<InvalidOperationException>();
+        }
+
         [Fact]
         public void Descending()
         {
@@ -84,6 +103,14 @@ public void Descending()
             Assert(subject.Descending("a"), "{a: -1}");
         }
 
+        [Fact]
+        public void Descending_no_field()
+        {
+            var subject = CreateSubject<BsonDocument>();
+
+            Assert(subject.Descending(), "{direction: -1}");
+        }
+
         [Fact]
         public void Descending_Typed()
         {