First pass of review polishing.

Original pull request: #3647.
Closes #3602.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/ReferenceLoader.java

@@ -35,8 +35,10 @@ public interface ReferenceLoader {
 		return bulkFetch(filter, context).findFirst().orElse(null);
 	}
 
+	// meh, Stream!
 	Stream<Document> bulkFetch(ReferenceFilter filter, ReferenceContext context);
 
+	// Reference query
 	interface ReferenceFilter {
 
 		Bson getFilter();
@@ -45,6 +47,8 @@ public interface ReferenceLoader {
 			return new Document();
 		}
 
+		// TODO: Move apply method into something else that holds the collection and knows about single item/multi-item
+		// processing
 		default Stream<Document> apply(MongoCollection<Document> collection) {
 			return restoreOrder(StreamSupport.stream(collection.find(getFilter()).sort(getSort()).spliterator(), false));
 		}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/ReferenceReader.java

@@ -80,6 +80,7 @@ public class ReferenceReader {
 		this.codec = new ParameterBindingDocumentCodec();
 	}
 
+	// TODO: Move documentConversionFunction to here. Having a contextual read allows projections in references
 	Object readReference(MongoPersistentProperty property, Object value,
 			BiFunction<ReferenceContext, ReferenceFilter, Stream<Document>> lookupFunction) {
 
@@ -94,6 +95,8 @@ public class ReferenceReader {
 			return result.map(it -> documentConversionFunction.apply(property, it)).collect(Collectors.toList());
 		}
 
+		// TODO: retain target type and extract types here so the conversion function doesn't require type fiddling
+		// BiFunction<TypeInformation, Document, Object> instead of MongoPersistentProperty
 		if (property.isMap()) {
 
 			// the order is a real problem here
@@ -165,7 +168,7 @@ public class ReferenceReader {
 
 		if (!BsonUtils.isJsonDocument(value) && value.contains("?#{")) {
 			String s = "{ 'target-value' : " + value + "}";
-			T evaluated = (T) new ParameterBindingDocumentCodec().decode(s, bindingContext).get("target-value ");
+			T evaluated = (T) codec.decode(s, bindingContext).get("target-value ");
 			return evaluated != null ? evaluated : defaultValue.get();
 		}
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/ReferenceResolver.java

@@ -47,6 +47,7 @@ public interface ReferenceResolver {
 
 	ReferenceLoader getReferenceLoader();
 
+	// TODO: ReferenceCollection
 	class ReferenceContext {
 
 		@Nullable final String database;

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/mapping/ObjectReference.java

@@ -19,6 +19,7 @@ package org.springframework.data.mongodb.core.mapping;
  * @author Christoph Strobl
  */
 @FunctionalInterface
+// TODO: ObjectPointer or DocumentPointer
 public interface ObjectReference<T> {
 	T getPointer();
 }