CSHARP-2739: Improve hierarchical fields projection.

Author: onyxmaster

src/MongoDB.Driver/Linq/Translators/FindProjectionTranslator.cs

@@ -218,12 +218,8 @@ private static IReadOnlyList<IFieldExpression> GetUniqueFieldsByHierarchy(IEnume
                 var referenceGroup = referenceGroups.Dequeue();
                 if (!skippedFields.Contains(referenceGroup.Key))
                 {
-                    var hierarchicalReferenceGroups = referenceGroups
-                        .Where(x =>
-                        {
-                            return x.Key.Count(c => c == '.') != referenceGroup.Key.Count(c => c == '.') && // otherwise, fields are on the same nesting level
-                                   x.Key.StartsWith(referenceGroup.Key);
-                        });
+                    var prefix = referenceGroup.Key + '.';
+                    var hierarchicalReferenceGroups = referenceGroups.Where(x => x.Key.StartsWith(prefix));
                     uniqueFields.AddRange(referenceGroup);
                     skippedFields.AddRange(hierarchicalReferenceGroups.Select(x => x.Key));
                 }

tests/MongoDB.Driver.Tests/Linq/Translators/FindProjectionTranslatorTests.cs

@@ -192,6 +192,17 @@ public void Should_translate_with_a_hierarchical_redundancy_and_a_top_level_proj
             result.Value.F.Should().Be(2);
         }
 
+        [Fact]
+        public void Should_translate_with_a_hierarchical_redundancy_and_when_one_field_doesnt_start_with_another()
+        {
+            var result = Project(p => new { p.C.E, F = p.C.E1.F }, "{ C : { E : { H : 3 }, E1 : { F : 2 } } }");
+
+            result.Projection.Should().Be("{ \"C.E\" : 1, \"C.E1.F\" : 1, _id : 0 }");
+
+            result.Value.E.H.Should().Be(3);
+            result.Value.F.Should().Be(2);
+        }
+
         [Fact]
         public void Should_translate_with_a_hierarchical_redundancy_and_when_one_field_starts_with_another()
         {