Fix issue with reference conversion in updates.

We now make sure to convert references in update operations targeting collection like fields when using eg. the push modifier.

Closes #4041
Original pull request: #4045.

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

@@ -15,8 +15,10 @@
  */
 package org.springframework.data.mongodb.core.convert;
 
+import java.util.ArrayList;
 import java.util.Collection;
 import java.util.LinkedHashMap;
+import java.util.List;
 import java.util.Map.Entry;
 
 import org.bson.Document;
@@ -34,6 +36,7 @@ import org.springframework.data.mongodb.core.query.Update.Modifiers;
 import org.springframework.data.util.ClassTypeInformation;
 import org.springframework.data.util.TypeInformation;
 import org.springframework.lang.Nullable;
+import org.springframework.util.ObjectUtils;
 
 /**
  * A subclass of {@link QueryMapper} that retains type information on the mongo types.
@@ -221,8 +224,18 @@ public class UpdateMapper extends QueryMapper {
 					: getMappedSort(sortObject, field.getPropertyEntity());
 		}
 
-		TypeInformation<?> typeHint = field == null ? ClassTypeInformation.OBJECT : field.getTypeHint();
+		if (isAssociationConversionNecessary(field, value)) {
+			if (ObjectUtils.isArray(value) || value instanceof Collection) {
+				List<Object> targetPointers = new ArrayList<>();
+				for (Object val : converter.getConversionService().convert(value, List.class)) {
+					targetPointers.add(getMappedValue(field, val));
+				}
+				return targetPointers;
+			}
+			return super.getMappedValue(field, value);
+		}
 
+		TypeInformation<?> typeHint = field == null ? ClassTypeInformation.OBJECT : field.getTypeHint();
 		return converter.convertToMongoType(value, typeHint);
 	}
 

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

@@ -903,6 +903,29 @@ public class MongoTemplateDocumentReferenceTests {
 		assertThat(target).containsEntry("toB", "b");
 	}
 
+	@Test // GH-4041
+	void updateReferenceWithPushToCollection() {
+
+		WithListOfRefs a = new WithListOfRefs();
+		a.id = "a";
+		template.save(a);
+
+		WithListOfRefs b = new WithListOfRefs();
+		b.id = "b";
+		template.save(b);
+
+		template.update(WithListOfRefs.class).matching(where("id").is(a.id))
+				.apply(new Update().push("refs").each(new Object[] { b })).first();
+
+		String collection = template.getCollectionName(WithListOfRefs.class);
+
+		Document target = template.execute(db -> {
+			return db.getCollection(collection).find(Filters.eq("_id", "a")).first();
+		});
+
+		assertThat(target).containsEntry("refs", Collections.singletonList("b"));
+	}
+
 	@Test // GH-3782
 	void updateReferenceHavingCustomizedIdTargetType() {
 
@@ -1584,4 +1607,11 @@ public class MongoTemplateDocumentReferenceTests {
 			return publisher;
 		}
 	}
+
+	@Data
+	public static class WithListOfRefs {
+		@Id private String id;
+
+		@DocumentReference private List<WithListOfRefs> refs;
+	}
 }