DATAMONGO-1831 - Fix array type conversion for empty source.

We now make sure that we convert empty sources to the corresponding target type. This prevents entity instantiation from failing due to incorrect argument types when invoking the constructor.

Original pull request: spring-projects#520.

Author: christophstrobl

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

@@ -954,7 +954,7 @@ private Object readCollectionOrArray(TypeInformation<?> targetType, List sourceV
 				: CollectionFactory.createCollection(collectionType, rawComponentType, sourceValue.size());
 
 		if (sourceValue.isEmpty()) {
-			return getPotentiallyConvertedSimpleRead(items, collectionType);
+			return getPotentiallyConvertedSimpleRead(items, targetType.getType());
 		}
 
 		if (!DBRef.class.equals(rawComponentType) && isCollectionOfDbRefWhereBulkFetchIsPossible(sourceValue)) {

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

@@ -21,6 +21,8 @@
 import static org.mockito.Mockito.*;
 import static org.springframework.data.mongodb.core.DocumentTestUtils.*;
 
+import lombok.RequiredArgsConstructor;
+
 import java.math.BigDecimal;
 import java.math.BigInteger;
 import java.net.URL;
@@ -1803,6 +1805,22 @@ public void failsReadingDocumentIntoSimpleType() {
 		converter.read(TypeWithMapOfLongValues.class, source);
 	}
 
+	@Test // DATAMONGO-1831
+	public void shouldConvertArrayInConstructorCorrectly() {
+
+		org.bson.Document source = new org.bson.Document("array", Collections.emptyList());
+
+		assertThat(converter.read(WithArrayInConstructor.class, source).array, is(emptyArray()));
+	}
+
+	@Test // DATAMONGO-1831
+	public void shouldConvertNullForArrayInConstructorCorrectly() {
+
+		org.bson.Document source = new org.bson.Document();
+
+		assertThat(converter.read(WithArrayInConstructor.class, source).array, is(nullValue()));
+	}
+
 	static class GenericType<T> {
 		T content;
 	}
@@ -2157,4 +2175,11 @@ static class TypeWithPropertyInNestedField {
 	static class TypeWithMapOfLongValues {
 		Map<String, Long> map;
 	}
+
+	@RequiredArgsConstructor
+	static class WithArrayInConstructor {
+
+		final String[] array;
+
+	}
 }