Why are Collection properties indexed in objectify, sometimes? - google-app-engine

Question

In objectify, when I define a collection property with String datatype,
#IgnoreSave(IfEmpty.class)
private Set<String> collectionProperty = new HashSet<>();
and then look at a record in datastore, it appears indexed even though I have not annotated it with #Index.
Contrary, when I use a complex Object instead String, it does not appear as indexed.
Why are Collection properties indexed sometimes and sometimes not? And is there a way to determine this?
--
Unmodified code and screenshot from admin console/datastore:
#Entity
#Cache(expirationSeconds = 900)
public class Item extends StringId implements Serializable {
private static final Logger log = Logger.getLogger(Item.class.getSimpleName());
private static final long serialVersionUID = 1;
// Constructors
private Item() {}
#Nonnull
private static Item create(#Nonnull String itemId) {
Item item = (Item) new Item().setId(itemId);
item.piecesFromId();
log.info("item = " + JsonHelper.logToJson(item));
return item;
}
#Nonnull
public static Item create(#Nonnull String provider, #Nonnull String type, #Nonnull String identifier) {
String itemId = IdHelper.createItemId(provider, type, identifier);
Item item = ((Item) new Item().setId(itemId))
.setProvider(provider)
.setType(type)
.setIdentifier(identifier);
log.info("item = " + JsonHelper.logToJson(item));
return item;
}
#Nonnull
public static Item loadOrCreate(#Nonnull String itemId) {
Item item = ofy().load().type(Item.class).id(itemId).now();
if (item == null) {
item = Item.create(itemId);
}
return item;
}
#Nullable
public static Item load(#Nonnull String itemId) {
return ofy().load().type(Item.class).id(itemId).now();
}
#OnLoad
private void piecesFromId() {
provider = IdHelper.getProvider(id);
type = IdHelper.getType(id);
identifier = IdHelper.getIdentifier(id);
}
public Item save() {
ofy().defer().save().entity(this);
return this;
}
#OnSave
private void integrity() {
if (id == null) { throw new RuntimeException("Id must not be null."); }
if (itemPreview == null) { throw new RuntimeException("itemPreview must not be null."); }
if (provider == null || type == null || identifier == null) { throw new RuntimeException("provider, type and identifier must not be null."); }
if (!id.equals(IdHelper.createItemId(provider, type, identifier))) { throw new RuntimeException("id does not coincide with provider, type and identifier."); }
if (!id.equals(itemPreview.getItemId())) { throw new RuntimeException("id does not coincide with id in itemPreview."); }
}
#OnSave
private void timestamp() {
if (created == null) {
created = System.currentTimeMillis();
}
}
// Properties
#Ignore
private String provider;
#Ignore
private String type;
#Ignore
private String identifier;
#Ignore // json
private ItemPreview itemPreview;
#ApiResourceProperty(ignored = AnnotationBoolean.TRUE)
#IgnoreSave(IfEmpty.class)
private Set<String> subscribedUserIds = new HashSet<>();
#ApiResourceProperty(ignored = AnnotationBoolean.TRUE)
#IgnoreSave(IfEmpty.class)
private Set<String> notifyUserIds = new HashSet<>();
#ApiResourceProperty(ignored = AnnotationBoolean.TRUE)
#IgnoreSave(IfEmpty.class)
private Set<String> blacklistingUserIds = new HashSet<>();
#ApiResourceProperty(ignored = AnnotationBoolean.TRUE)
private Long created;
#ApiResourceProperty(ignored = AnnotationBoolean.TRUE)
#IgnoreSave(IfDefault.class)
#Index
private Status status = Status.ACTIVE;
#ApiResourceProperty(ignored = AnnotationBoolean.TRUE)
#IgnoreSave(IfNull.class)
private String suspensionNotice;
// Json
#ApiResourceProperty(ignored = AnnotationBoolean.TRUE)
#IgnoreSave(IfNull.class)
private String itemPreviewJson;
private static Type itemPreviewType = new TypeToken<ItemPreview>(){}.getType();
#OnLoad
private void itemPreviewFromJson() {
if (itemPreviewJson != null) {
itemPreview = new GsonBuilder().excludeFieldsWithoutExposeAnnotation().create()
.fromJson(itemPreviewJson, itemPreviewType);
}
}
#OnSave
private void itemPreviewToJson() {
itemPreviewJson = new GsonBuilder().excludeFieldsWithoutExposeAnnotation().create()
.toJson(itemPreview, itemPreviewType);
}
// Accessors
public String getProvider() {
return provider;
}
public Item setProvider(String provider) {
this.provider = provider;
return this;
}
public String getType() {
return type;
}
public Item setType(String type) {
this.type = type;
return this;
}
public String getIdentifier() {
return identifier;
}
public Item setIdentifier(String identifier) {
this.identifier = identifier;
return this;
}
public ItemPreview getItemPreview() {
return itemPreview;
}
public Item setItemPreview(ItemPreview itemPreview) {
this.itemPreview = itemPreview;
return this;
}
public Set<String> getSubscribedUserIds() {
return subscribedUserIds;
}
public Item setSubscribedUserIds(Set<String> subscribedUserIds) {
this.subscribedUserIds = subscribedUserIds;
return this;
}
public Set<String> getNotifyUserIds() {
return notifyUserIds;
}
public Item setNotifyUserIds(Set<String> notifyUserIds) {
this.notifyUserIds = notifyUserIds;
return this;
}
public Set<String> getBlacklistingUserIds() {
return blacklistingUserIds;
}
public Item setBlacklistingUserIds(Set<String> blacklistingUserIds) {
this.blacklistingUserIds = blacklistingUserIds;
return this;
}
public Long getCreated() {
return created;
}
public Item setCreated(Long created) {
this.created = created;
return this;
}
public Status getStatus() {
return status;
}
public Item setStatus(Status status) {
this.status = status;
return this;
}
public String getSuspensionNotice() {
return suspensionNotice;
}
public Item setSuspensionNotice(String suspensionNotice) {
this.suspensionNotice = suspensionNotice;
return this;
}
// Collections
public static Map<String, Item> loadAll(Set<String> itemIds) {
return ofy().load().type(Item.class).ids(itemIds);
}
}