Apply conversion on document reference lookup using nested property.

Closes #4033
Original pull request: #4044.

Author: christophstrobl

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

@@ -373,7 +373,9 @@ protected Document getMappedKeyword(Keyword keyword, @Nullable MongoPersistentEn
 		if (keyword.isOrOrNor() || (keyword.hasIterableValue() && !keyword.isGeometry())) {
 
 			Iterable<?> conditions = keyword.getValue();
-			List<Object> newConditions = conditions instanceof Collection ? new ArrayList<>(((Collection<?>) conditions).size()) : new ArrayList<>();
+			List<Object> newConditions = conditions instanceof Collection
+					? new ArrayList<>(((Collection<?>) conditions).size())
+					: new ArrayList<>();
 
 			for (Object condition : conditions) {
 				newConditions.add(isDocument(condition) ? getMappedObject((Document) condition, entity)
@@ -431,8 +433,10 @@ protected Object getMappedValue(Field documentField, Object sourceValue) {
 
 		Object value = applyFieldTargetTypeHintToValue(documentField, sourceValue);
 
-		if(documentField.getProperty() != null && converter.getCustomConversions().hasValueConverter(documentField.getProperty())) {
-			return converter.getCustomConversions().getPropertyValueConversions().getValueConverter(documentField.getProperty())
+		if (documentField.getProperty() != null
+				&& converter.getCustomConversions().hasValueConverter(documentField.getProperty())) {
+			return converter.getCustomConversions().getPropertyValueConversions()
+					.getValueConverter(documentField.getProperty())
 					.write(value, new MongoConversionContext(documentField.getProperty(), converter));
 		}
 
@@ -616,7 +620,11 @@ protected Object delegateConvertToMongoType(Object source, @Nullable MongoPersis
 	}
 
 	protected Object convertAssociation(Object source, Field field) {
-		return convertAssociation(source, field.getProperty());
+		Object value = convertAssociation(source, field.getProperty());
+		if (value != null && field.isIdField() && field.getFieldType() != value.getClass()) {
+			return convertId(value, field.getFieldType());
+		}
+		return value;
 	}
 
 	/**
@@ -1042,6 +1050,9 @@ public TypeInformation<?> getTypeHint() {
 			return TypeInformation.OBJECT;
 		}
 
+		public Class<?> getFieldType() {
+			return Object.class;
+		}
 	}
 
 	/**
@@ -1170,6 +1181,11 @@ private Association<MongoPersistentProperty> findAssociation() {
 			return null;
 		}
 
+		@Override
+		public Class<?> getFieldType() {
+			return property.getFieldType();
+		}
+
 		@Override
 		public String getMappedKey() {
 			return path == null ? name : path.toDotPath(isAssociation() ? getAssociationConverter() : getPropertyConverter());

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/QueryMapperUnitTests.java

@@ -437,6 +437,31 @@ void convertsDocumentReferenceOnIdPropertyCorrectly() {
 		assertThat(mappedQuery).containsEntry("sample", "s1");
 	}
 
+	@Test // GH-4033
+	void convertsNestedPathToIdPropertyOfDocumentReferenceCorrectly() {
+
+		Query query = query(where("sample.foo").is("s1"));
+		org.bson.Document mappedQuery = mapper.getMappedObject(query.getQueryObject(),
+				context.getPersistentEntity(WithDocumentReference.class));
+
+		assertThat(mappedQuery).containsEntry("sample", "s1");
+	}
+
+	@Test // GH-4033
+	void convertsNestedPathToIdPropertyOfDocumentReferenceCorrectlyWhenItShouldBeConvertedToObjectId() {
+
+		ObjectId id = new ObjectId();
+		Query query = query(where("sample.foo").is(id.toHexString()));
+		org.bson.Document mappedQuery = mapper.getMappedObject(query.getQueryObject(),
+				context.getPersistentEntity(WithDocumentReference.class));
+
+		assertThat(mappedQuery.get("sample")).satisfies(it -> {
+
+			assertThat(it).isInstanceOf(ObjectId.class);
+			assertThat(((ObjectId) it).toHexString()).isEqualTo(id.toHexString());
+		});
+	}
+
 	@Test // GH-3853
 	void convertsListDocumentReferenceOnIdPropertyCorrectly() {