Add an option to @Field annotation to include/exclude null values on write.

Properties can be annotated with `@Field(write=…)` to control whether a property with a null value should be included or omitted (default) during conversion in the target Document.

Closes #3407
Original pull request: #3646.

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

@@ -744,6 +744,9 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 			Object value = accessor.getProperty(prop);
 
 			if (value == null) {
+				if(!prop.isPropertyOmittableOnNull()) {
+					writeSimpleInternal(value, bson , prop);
+				}
 				continue;
 			}
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/mapping/BasicMongoPersistentProperty.java

@@ -285,4 +285,18 @@ public class BasicMongoPersistentProperty extends AnnotationBasedPersistentPrope
 	public boolean isTextScoreProperty() {
 		return isAnnotationPresent(TextScore.class);
 	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.mapping.MongoPersistentProperty#isPropertyOmittableOnNull()
+	 */
+	public boolean isPropertyOmittableOnNull() {
+		org.springframework.data.mongodb.core.mapping.Field annotation = findAnnotation(
+				org.springframework.data.mongodb.core.mapping.Field.class);
+
+		if ( annotation != null && annotation.write().equals(Field.Write.ALWAYS) ) {
+			return false;
+		}
+		return true;
+	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/mapping/Field.java

@@ -34,6 +34,21 @@ import org.springframework.core.annotation.AliasFor;
 @Target({ ElementType.FIELD, ElementType.METHOD, ElementType.ANNOTATION_TYPE })
 public @interface Field {
 
+	/**
+	 * Enumeration of write strategies for a field with null value.It decides whether a field with null value has to be
+	 * written to the resulting document to be saved to the database.
+	 */
+	enum Write{
+		/*
+		 * The field will always be written to the database irrespective of null value.
+		 */
+		ALWAYS,
+		/*
+		 * The field will only be written to the database if it has a non null value.
+		 */
+		NON_NULL
+	}
+
 	/**
 	 * The key to be used to store the field inside the document. Alias for {@link #name()}.
 	 *
@@ -65,4 +80,14 @@ public @interface Field {
 	 * @since 2.2
 	 */
 	FieldType targetType() default FieldType.IMPLICIT;
+
+	/**
+	 * If set to {@link Write#NON_NULL} {@literal null} values will be omitted.
+	 * Setting the value to {@link Write#ALWAYS} explicitly adds an entry for the given field
+	 * holding {@literal null} as a value {@code 'fieldName' : null }.
+	 * <p />
+	 * <strong>NOTE</strong> Setting the value to {@link Write#ALWAYS} may lead to increased document size.
+	 * @return {@link Write#NON_NULL} by default.
+	 */
+	Write write() default Write.NON_NULL;
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/mapping/MongoPersistentProperty.java

@@ -104,6 +104,24 @@ public interface MongoPersistentProperty extends PersistentProperty<MongoPersist
 	 * @since 1.6
 	 */
 	boolean isTextScoreProperty();
+	
+	/**
+	 * Returns whether the property is to be written to the document if the value is null <br/>
+	 * It's annotated with {@link Field.Write}.
+	 *
+	 * @return
+	 * @since 1.6
+	 */
+	boolean isPropertyOmittableOnNull(); 
+
+	/**
+	 * Returns whether the property is to be written to the document if the value is null <br/>
+	 * It's annotated with {@link omitNull}.
+	 *
+	 * @return
+	 * @since 1.6
+	 */
+	boolean isOmitNullProperty(); 
 
 	/**
 	 * Returns the {@link DBRef} if the property is a reference.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/mapping/UnwrappedMongoPersistentProperty.java

@@ -92,6 +92,11 @@ class UnwrappedMongoPersistentProperty implements MongoPersistentProperty {
 	public boolean isTextScoreProperty() {
 		return delegate.isTextScoreProperty();
 	}
+	
+	@Override
+	public boolean isOmitNullProperty() {
+		return delegate.isOmitNullProperty();
+	}
 
 	@Override
 	@Nullable
@@ -315,4 +320,9 @@ class UnwrappedMongoPersistentProperty implements MongoPersistentProperty {
 	public <T> PersistentPropertyAccessor<T> getAccessorForOwner(T owner) {
 		return delegate.getAccessorForOwner(owner);
 	}
+
+	@Override
+	public boolean isPropertyOmittableOnNull() {
+		return delegate.isPropertyOmittableOnNull();
+	}
 }

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/mapping/BasicMongoPersistentPropertyUnitTests.java

@@ -146,6 +146,13 @@ public class BasicMongoPersistentPropertyUnitTests {
 		assertThat(property.isTextScoreProperty()).isTrue();
 	}
 
+	@Test // DATAMONGO-2551
+	public void shouldDetectOmittableOnNullPropertyCorrectly() {
+
+		MongoPersistentProperty property = getPropertyFor(DocumentWithOmittableOnNullProperty.class, "write");
+		assertThat(property.isPropertyOmittableOnNull()).isTrue();
+	}
+
 	@Test // DATAMONGO-976
 	public void shouldDetectTextScoreAsReadOnlyProperty() {
 
@@ -297,6 +304,12 @@ public class BasicMongoPersistentPropertyUnitTests {
 		@TextScore Float score;
 	}
 
+	static class DocumentWithOmittableOnNullProperty {
+
+		@org.springframework.data.mongodb.core.mapping.Field("write") org.springframework.data.mongodb.core.mapping.Field.Write write;
+
+	}
+
 	static class DocumentWithExplicitlyRenamedIdProperty {
 
 		@org.springframework.data.mongodb.core.mapping.Field("id") String id;