CSHARP-1194: Refactor MongoCollectionImpl OfType method to use new OfType FilterDefinitionBuilder method.

Author: rstam

src/MongoDB.Driver/FilterDefinitionBuilder.cs

@@ -1598,17 +1598,25 @@ public override BsonDocument Render(IBsonSerializer<TDocument> documentSerialize
             }
 
             var discriminatorValue = discriminatorConvention.GetDiscriminator(typeof(TDocument), typeof(TDerived));
-            var renderedDiscriminatorFilter = new BsonDocument(discriminatorConvention.ElementName, discriminatorValue);
+            if (discriminatorValue == null)
+            {
+                throw new NotSupportedException($"OfType requires that documents of type {BsonUtils.GetFriendlyTypeName(typeof(TDerived))} have a discriminator value.");
+            }
+            if (discriminatorValue.IsBsonArray)
+            {
+                discriminatorValue = discriminatorValue.AsBsonArray.Last();
+            }
+            var renderedOfTypeFilter = new BsonDocument(discriminatorConvention.ElementName, discriminatorValue);
 
             if (_derivedDocumentFilter == null)
             {
-                return renderedDiscriminatorFilter;
+                return renderedOfTypeFilter;
             }
 
             var derivedDocumentSerializer = serializerRegistry.GetSerializer<TDerived>();
             var renderedDerivedDocumentFilter = _derivedDocumentFilter.Render(derivedDocumentSerializer, serializerRegistry);
             var combinedFilter = Builders<TDerived>.Filter.And(
-                new BsonDocumentFilterDefinition<TDerived>(renderedDiscriminatorFilter),
+                new BsonDocumentFilterDefinition<TDerived>(renderedOfTypeFilter),
                 new BsonDocumentFilterDefinition<TDerived>(renderedDerivedDocumentFilter));
             return combinedFilter.Render(derivedDocumentSerializer, serializerRegistry);
         }

src/MongoDB.Driver/MongoCollectionImpl.cs

@@ -398,24 +398,9 @@ public override IFilteredMongoCollection<TDerivedDocument> OfType<TDerivedDocume
             var ofTypeSerializer = new OfTypeSerializer<TDocument, TDerivedDocument>(derivedDocumentSerializer);
             var derivedDocumentCollection = new MongoCollectionImpl<TDerivedDocument>(_database, _collectionNamespace, _settings, _cluster, _operationExecutor, ofTypeSerializer);
 
-            var discriminatorConvention = BsonSerializer.LookupDiscriminatorConvention(typeof(TDocument));
-            var discriminator = discriminatorConvention.GetDiscriminator(typeof(TDocument), typeof(TDerivedDocument));
-            if (discriminator == null)
-            {
-                throw new NotSupportedException("OfType requires that the root document type have a discriminator.");
-            }
-
-            BsonValue filterValue;
-            if (discriminator.IsBsonArray)
-            {
-                filterValue = discriminator[discriminator.AsBsonArray.Count - 1];
-            }
-            else
-            {
-                filterValue = discriminator;
-            }
-            var filterDocument = new BsonDocument(discriminatorConvention.ElementName, filterValue);
-            var ofTypeFilter = new BsonDocumentFilterDefinition<TDerivedDocument>(filterDocument);
+            var rootOfTypeFilter = Builders<TDocument>.Filter.OfType<TDerivedDocument>();
+            var renderedOfTypeFilter = rootOfTypeFilter.Render(_documentSerializer, _settings.SerializerRegistry);
+            var ofTypeFilter = new BsonDocumentFilterDefinition<TDerivedDocument>(renderedOfTypeFilter);
 
             return new OfTypeMongoCollection<TDocument, TDerivedDocument>(this, derivedDocumentCollection, ofTypeFilter);
         }