Load list of items in objectify - google-app-engine

Question

I have Question, Like and Hashtag entities. Also there is one to many relationship between Like and Question entities. I am using google cloud endpoints and my problem begins here. In my list method, I return 20 question as json. But for each question object in query I have to check if user is already liked the question and also fetch related hashtags that belongs to the question. How can I do the same operation by key only batch query. Otherwise, I do
ofy().load().type(Like.class)
.filter("questionRef =", questionKey)
.filter("accountRef =", accountKey).first().now();
for each object.
Like entity
#Entity
#Cache
public class Like {
#Id
#Getter
protected Long id;
#Index
#Load
#ApiResourceProperty(ignored = AnnotationBoolean.TRUE)
private Ref<Account> accountRef;
#Index
#Load
#ApiResourceProperty(ignored = AnnotationBoolean.TRUE)
private Ref<Question> questionRef;
#Index
#Getter
protected Date createdAt;
Like() {
}
Like(Key<Account> accountKey) {
this.accountRef = Ref.create(accountKey);
this.createdAt = new Date();
}
}
Hashtag entity
#Entity
#Cache
public class Hashtag implements Model<Hashtag> {
#Id
#Getter
#ApiResourceProperty(ignored = AnnotationBoolean.TRUE)
private Long id;
#Index
#Load
#ApiResourceProperty(ignored = AnnotationBoolean.TRUE)
private Ref<Question> questionRef;
#Index
#Getter
#Setter
private String text;
private Hashtag() {
}
private Hashtag(Builder builder) {
this.questionRef = builder.questionRef;
this.text = builder.text;
}
}

Answer 1

There are several parts to this question.
First, hashtags: Just store hashtags in the Question as an indexed list property. Easy.
Second, likes: There are a couple ways to do this efficiently.
One is to create a Like entity with a natural key of "account:question" (use the stringified websafe key). This way you can do a batch get by key for all the {user,question} tuples. Some will be absent, some will be present. Reasonably efficient if you're only concerned about 20 questions, especially if you #Cache the Like.
Another is to create a separate Relation Index Entity that tracks all the likes of a user and just load those up each time. You can put 5k items in any list property, which means you'll need to juggle multiple entities when a user likes more than 5k things. But it's easy to load them all up with a single ancestor query. The RIE will need to be #Parented by the User.
On a separate note - don't call fields thingRef. It's just a thing. The data in the database is just a key. You can interchange Ref<?>, Key<?>, and the native low-level Key. Type information doesn't belong in database names.

Answer 2

I am not sure if you can change the structure of your entities. If the answer is no, then there is no option other than the approach you have taken.
If yes, I would suggest structuring your Question to include the Like and Hashtag information as well.
#Entity
public class Question {
#Id
private long id;
private Set<Key<Account>> likedBy;
private List<String> hashtags;
}
For a question, you can retrieve all the information in one single query. Then collect all the Account keys and make another datastore query to retrieve all the people who have liked the question using keys as below:
Map<Key<Account>, Account> likedByAccounts = ofy().load().keys(accountKeys);