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: #520.

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

@@ -904,7 +904,7 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 				: 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.junit.Assert.*;
 import static org.mockito.Mockito.*;
 import static org.springframework.data.mongodb.core.DBObjectTestUtils.*;
 
+import lombok.RequiredArgsConstructor;
+
 import java.math.BigDecimal;
 import java.math.BigInteger;
 import java.net.URL;
@@ -1797,6 +1799,22 @@ public class MappingMongoConverterUnitTests {
 		assertThat(converter.read(ClassWithEnumProperty.class, source).enumSet, is(EnumSet.noneOf(SampleEnum.class)));
 	}
 
+	@Test // DATAMONGO-1831
+	public void shouldConvertArrayInConstructorCorrectly() {
+
+		DBObject source = new BasicDBObject("array", Collections.emptyList());
+
+		assertThat(converter.read(WithArrayInConstructor.class, source).array, is(emptyArray()));
+	}
+
+	@Test // DATAMONGO-1831
+	public void shouldConvertNullForArrayInConstructorCorrectly() {
+
+		DBObject source = new BasicDBObject();
+
+		assertThat(converter.read(WithArrayInConstructor.class, source).array, is(nullValue()));
+	}
+
 	static class GenericType<T> {
 		T content;
 	}
@@ -2147,4 +2165,11 @@ public class MappingMongoConverterUnitTests {
 	static class TypeWithPropertyInNestedField {
 		@Field("nested.sample") String sample;
 	}
+
+	@RequiredArgsConstructor
+	static class WithArrayInConstructor {
+
+		final String[] array;
+
+	}
 }