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

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

@@ -38,6 +38,7 @@ import org.bson.codecs.DecoderContext;
 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 class MappingMongoConverter extends AbstractMongoConverter implements App
 
 			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 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 		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 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 		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 class MappingMongoConverter extends AbstractMongoConverter implements App
 				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) {

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

@@ -44,6 +44,7 @@ import org.junit.jupiter.api.extension.ExtendWith;
 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 @@ class MappingMongoConverterUnitTests {
 		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 @@ class MappingMongoConverterUnitTests {
 		String getName();
 	}
 
+	@lombok.Value
+	static class AuthorOnly {
+
+		AuthorNameOnly author;
+	}
+
+	@lombok.Value
+	static class AuthorNameOnly {
+
+		String firstName;
+
+		String lastName;
+
+	}
+
 	@Data
 	static class Book {