Morphia. How to get a part of information from big datastore - spring

Question

I have some problem with Morphia.
Could someone help me?
I am writing web-project on Spring + MongoDB about movies and celebrities.
I have entity class Genre:
#Entity(value="genres")
public class Genre implements IGenre {
#Id
#Indexed
private ObjectId id;
#Indexed
private String name;
private String description;
private long quantity;
private Set <IMovie> movies;
//getters and setters
}
And entity class Movie:
#Entity(value="movies")
public class Movie implements IMovie {
#Id
#Indexed
private ObjectId id;
#Indexed
private String originalTitle;
private String year;
private Set <IGenre> genres;
// getters and setters
}
I have 30 genres. And for example one of them: Comedy.
Also I have 250 000 comedies.
And now I want to do movie pagination by genre = comedy.
How I can get only 20 records from all comedies.
If I use #Embedded or #Reference annotation I will still get the entire list at once. And it's to big for use it in controllers.

Answer 1

You should change your data schema for doing such a query. The schema you use has a circular dependency, in your Genre entity you are holding Movies entity and in movies you hold Genre. Also holding all of the movies according to genre is not easy to query. If I were you I would use such a schema.
#Entity(noClassnameStored = true) // you wouldn't have any problem when you change the class name or package of your class if you don't store the classname through this annotation
public class Movie implements IMovie {
#Id
#Indexed
private ObjectId id;
#Indexed
private String originalTitle;
private String year;
private Set <String> genres; // unique identifier of the genres instead of embedding the whole genre entity
// getters and setters
}
So having such a schema, you can retrieve the movies having a particular genre by writing a simple $in query through genres field. Example query for your case:
datastore.find(Movie.class).field("genres").in(Lists.newArrayList("comedy")).limit(20).asList;
In the below web-page of mongo you can find suggestions about how to design your schema according to diffrerent scenarios.
http://docs.mongodb.org/manual/core/data-modeling/#data-modeling-patterns-and-examples

Answer 2

I'm not too familiar with Mongo, but it looks like you would need to implement a custom query here. Whatever you do, you need to pass a start for your page, as well as a page size (20 in your case).
You can do pagination in Morphia by combining .offset(page_start) and .limit(page_size) on a query. So first you would create a query to get movies that belong to a certain genre, and then apply the pagination.
Looks like it is handled using repositories in Spring http://static.springsource.org/spring-data/data-mongodb/docs/1.0.0.RELEASE/reference/html/#repositories.special-parameters
You'd use a Pageable implementation to pass paging data, without worrying about doing the offset and limit calls yourself. There's an example of "Web pagination" further in the doc.
Hope this helps!