Fix DTO projection instantiation.

We now correctly instantiate DTO projection classes by using the actual constructor argument type. Previously, we did not update the conversion context to fetch the correct type but used the type of the DTO projection class instead of the constructor argument.

Closes #4120

Author: mp911de

Diff for: spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/MappingMongoConverter.java

@@ -38,6 +38,7 @@
 import org.bson.conversions.Bson;
 import org.bson.json.JsonReader;
 import org.bson.types.ObjectId;
+
 import org.springframework.beans.BeansException;
 import org.springframework.beans.factory.BeanClassLoaderAware;
 import org.springframework.context.ApplicationContext;
@@ -415,8 +416,8 @@ public ConversionContext forProperty(String name) {
 
 			EntityProjection<?, ?> property = returnedTypeDescriptor.findProperty(name);
 			if (property == null) {
-				return new ConversionContext(sourceConverter, conversions, path, MappingMongoConverter.this::readDocument, collectionConverter,
-						mapConverter, dbRefConverter, elementConverter);
+				return new ConversionContext(sourceConverter, conversions, path, MappingMongoConverter.this::readDocument,
+						collectionConverter, mapConverter, dbRefConverter, elementConverter);
 			}
 
 			return new ProjectingConversionContext(sourceConverter, conversions, path, collectionConverter, mapConverter,
@@ -966,8 +967,8 @@ protected void writePropertyInternal(@Nullable Object obj, DocumentAccessor acce
 		TypeInformation<?> type = prop.getTypeInformation();
 
 		if (conversions.getPropertyValueConversions().hasValueConverter(prop)) {
-			accessor.put(prop,
-					conversions.getPropertyValueConversions().getValueConverter(prop).write(obj, new MongoConversionContext(prop, this)));
+			accessor.put(prop, conversions.getPropertyValueConversions().getValueConverter(prop).write(obj,
+					new MongoConversionContext(prop, this)));
 			return;
 		}
 
@@ -1302,9 +1303,8 @@ private void writeSimpleInternal(@Nullable Object value, Bson bson, MongoPersist
 		DocumentAccessor accessor = new DocumentAccessor(bson);
 
 		if (conversions.getPropertyValueConversions().hasValueConverter(property)) {
-			accessor.put(property,
-					conversions.getPropertyValueConversions().getValueConverter(property)
-							.write(value, new MongoConversionContext(property, this)));
+			accessor.put(property, conversions.getPropertyValueConversions().getValueConverter(property).write(value,
+					new MongoConversionContext(property, this)));
 			return;
 		}
 
@@ -1971,12 +1971,15 @@ public <T> T getPropertyValue(MongoPersistentProperty property) {
 				return null;
 			}
 
-			if (context.conversions.getPropertyValueConversions().hasValueConverter(property)) {
-				return (T) context.conversions.getPropertyValueConversions().getValueConverter(property).read(value,
+			CustomConversions conversions = context.conversions;
+			if (conversions.getPropertyValueConversions().hasValueConverter(property)) {
+				return (T) conversions.getPropertyValueConversions().getValueConverter(property).read(value,
 						new MongoConversionContext(property, context.sourceConverter));
 			}
 
-			return (T) context.convert(value, property.getTypeInformation());
+			ConversionContext contextToUse = context.forProperty(property.getName());
+
+			return (T) contextToUse.convert(value, property.getTypeInformation());
 		}
 
 		public MongoDbPropertyValueProvider withContext(ConversionContext context) {

Diff for: spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/MappingMongoConverterUnitTests.java

@@ -44,6 +44,7 @@
 import org.mockito.Mock;
 import org.mockito.Mockito;
 import org.mockito.junit.jupiter.MockitoExtension;
+
 import org.springframework.aop.framework.ProxyFactory;
 import org.springframework.beans.ConversionNotSupportedException;
 import org.springframework.beans.factory.annotation.Autowired;
@@ -2734,6 +2735,23 @@ void shouldReadOpenProjection() {
 		assertThat(projection.getName()).isEqualTo("my-book by Walter White");
 	}
 
+	@Test // GH-4120
+	void shouldReadDtoProjection() {
+
+		org.bson.Document author = new org.bson.Document("firstName", "Walter").append("lastName", "White");
+		org.bson.Document book = new org.bson.Document("_id", "foo").append("name", "my-book").append("author", author);
+
+		EntityProjectionIntrospector introspector = EntityProjectionIntrospector.create(converter.getProjectionFactory(),
+				EntityProjectionIntrospector.ProjectionPredicate.typeHierarchy()
+						.and((target, underlyingType) -> !converter.conversions.isSimpleType(target)),
+				mappingContext);
+
+		AuthorOnly projection = converter.project(introspector.introspect(AuthorOnly.class, Book.class), book);
+
+		assertThat(projection.getAuthor().getFirstName()).isEqualTo("Walter");
+		assertThat(projection.getAuthor().getLastName()).isEqualTo("White");
+	}
+
 	@Test // GH-3596
 	void simpleConverter() {
 
@@ -3637,6 +3655,21 @@ interface BookProjection {
 		String getName();
 	}
 
+	@lombok.Value
+	static class AuthorOnly {
+
+		AuthorNameOnly author;
+	}
+
+	@lombok.Value
+	static class AuthorNameOnly {
+
+		String firstName;
+
+		String lastName;
+
+	}
+
 	@Data
 	static class Book {