Document list/map/set initialization on read.

pom.xml

@@ -5,7 +5,7 @@
 
 	<groupId>org.springframework.data</groupId>
 	<artifactId>spring-data-mongodb-parent</artifactId>
-	<version>4.3.0-SNAPSHOT</version>
+	<version>4.3.x-4571-SNAPSHOT</version>
 	<packaging>pom</packaging>
 
 	<name>Spring Data MongoDB</name>

spring-data-mongodb-benchmarks/pom.xml

@@ -7,7 +7,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>4.3.0-SNAPSHOT</version>
+		<version>4.3.x-4571-SNAPSHOT</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

spring-data-mongodb-distribution/pom.xml

@@ -15,7 +15,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>4.3.0-SNAPSHOT</version>
+		<version>4.3.x-4571-SNAPSHOT</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

spring-data-mongodb/pom.xml

@@ -13,7 +13,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>4.3.0-SNAPSHOT</version>
+		<version>4.3.x-4571-SNAPSHOT</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/util/BsonUtils.java

@@ -593,7 +593,7 @@ public static boolean hasValue(Bson bson, FieldName fieldName) {
 
 		Map<String, Object> source = asMap(bson);
 		if (fieldName.isKey()) {
-			return source.get(fieldName.name()) != null;
+			return source.containsKey(fieldName.name());
 		}
 
 		String[] parts = fieldName.parts();

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

@@ -27,6 +27,8 @@
 import java.time.LocalDateTime;
 import java.time.temporal.ChronoUnit;
 import java.util.*;
+import java.util.function.Function;
+import java.util.stream.Stream;
 
 import org.bson.BsonUndefined;
 import org.bson.types.Binary;
@@ -38,6 +40,8 @@
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
 import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
 import org.junit.jupiter.params.provider.ValueSource;
 import org.mockito.Mock;
 import org.mockito.Mockito;
@@ -673,6 +677,15 @@ void readsListOfMapsCorrectly() {
 		assertThat(wrapper.listOfMaps.get(0).get("Foo")).isEqualTo(Locale.ENGLISH);
 	}
 
+	@ParameterizedTest // GH-4571
+	@MethodSource("listMapSetReadingSource")
+	<T> void initializesListMapSetPropertiesIfRequiredOnRead(org.bson.Document source, Class<T> type,
+			Function<T, Object> valueFunction, Object expectedValue) {
+
+		T target = converter.read(type, source);
+		assertThat(target).extracting(valueFunction).isEqualTo(expectedValue);
+	}
+
 	@Test // DATAMONGO-259
 	void writesPlainMapOfCollectionsCorrectly() {
 
@@ -2986,6 +2999,49 @@ org.bson.Document write(Object source) {
 		return target;
 	}
 
+	private static Stream<Arguments> listMapSetReadingSource() {
+
+		Function<CollectionWrapper, Object> contacts = CollectionWrapper::getContacts;
+		Function<CollectionWrapper, Object> contactsSet = CollectionWrapper::getContactsSet;
+		Function<CollectionWrapper, Object> autoInitList = CollectionWrapper::getAutoInitList;
+		Function<ClassWithMapProperty, Object> map = ClassWithMapProperty::getMap;
+		Function<ClassWithMapProperty, Object> autoInitMap = ClassWithMapProperty::getAutoInitMap;
+
+		return Stream.of( //
+
+				// List
+				Arguments.of(new org.bson.Document("contacts", Collections.emptyList()), CollectionWrapper.class, contacts,
+						Collections.emptyList()),
+				Arguments.of(new org.bson.Document("contacts", null), CollectionWrapper.class, contacts, null),
+				Arguments.of(new org.bson.Document(), CollectionWrapper.class, contacts, null),
+
+				// ctor initialized List
+				Arguments.of(new org.bson.Document("autoInitList", Collections.emptyList()), CollectionWrapper.class,
+						autoInitList, Collections.emptyList()),
+				Arguments.of(new org.bson.Document("autoInitList", null), CollectionWrapper.class, autoInitList, null),
+				Arguments.of(new org.bson.Document(), CollectionWrapper.class, autoInitList,
+						Collections.singletonList("spring")),
+
+				// Set
+				Arguments.of(new org.bson.Document("contactsSet", Collections.emptyList()), CollectionWrapper.class,
+						contactsSet, Collections.emptySet()),
+				Arguments.of(new org.bson.Document("contactsSet", null), CollectionWrapper.class, contactsSet, null),
+				Arguments.of(new org.bson.Document(), CollectionWrapper.class, contactsSet, null),
+
+				// Map
+				Arguments.of(new org.bson.Document("map", new org.bson.Document()), ClassWithMapProperty.class, map,
+						Collections.emptyMap()),
+				Arguments.of(new org.bson.Document("map", null), ClassWithMapProperty.class, map, null),
+				Arguments.of(new org.bson.Document(), ClassWithMapProperty.class, map, null),
+
+				// ctor initialized Map
+				Arguments.of(new org.bson.Document("autoInitMap", new org.bson.Document()), ClassWithMapProperty.class,
+						autoInitMap, Collections.emptyMap()),
+				Arguments.of(new org.bson.Document("autoInitMap", null), ClassWithMapProperty.class, autoInitMap, null),
+				Arguments.of(new org.bson.Document(), ClassWithMapProperty.class, autoInitMap,
+						Collections.singletonMap("spring", "data")));
+	}
+
 	static class GenericType<T> {
 		T content;
 	}
@@ -3142,11 +3198,20 @@ static class ClassWithSortedMap {
 
 	static class ClassWithMapProperty {
 		Map<Locale, String> map;
+		Map<String, String> autoInitMap = Collections.singletonMap("spring", "data");
 		Map<String, List<String>> mapOfLists;
 		Map<String, Object> mapOfObjects;
 		Map<String, String[]> mapOfStrings;
 		Map<String, Person> mapOfPersons;
 		TreeMap<String, Person> treeMapOfPersons;
+
+		public Map<Locale, String> getMap() {
+			return map;
+		}
+
+		public Map<String, String> getAutoInitMap() {
+			return this.autoInitMap;
+		}
 	}
 
 	static class ClassWithNestedMaps {
@@ -3168,6 +3233,19 @@ static class CollectionWrapper {
 		List<List<String>> strings;
 		List<Map<String, Locale>> listOfMaps;
 		Set<Contact> contactsSet;
+		List<String> autoInitList = Collections.singletonList("spring");
+
+		public List<Contact> getContacts() {
+			return contacts;
+		}
+
+		public Set<Contact> getContactsSet() {
+			return contactsSet;
+		}
+
+		public List<String> getAutoInitList() {
+			return autoInitList;
+		}
 	}
 
 	static class LocaleWrapper {

src/main/antora/modules/ROOT/pages/mongodb/mapping/mapping.adoc

@@ -274,6 +274,17 @@ calling `get()` before the actual conversion
 |===
 ====
 
+.Collection Handling
+[NOTE]
+====
+Collection handing depends on the actual values retrieved from the MongoDB.
+
+* If a document does **not** contain the field mapped to a collection, the mapping will not touch the property.
+Which means the value will remain `null`, a java default or any value set during object creation.
+* If the document contains the field to be mapped, but the field holds a `null` value (like: `{ 'list' : null }`), the property value is set to `null` overriding any default value set during object creation.
+* If the document contains the field to be mapped to a collection which is **not** `null` (like: `{ 'list' : [ ... ] }`), the collection is populated with the mapped values overriding any default value set during object creation.
+====
+
 [[mapping-configuration]]
 == Mapping Configuration