Reviews : Cultivating your relationship with Castor-JDO :

Lazy loading

When you're working with relations between your classes Castor will automatically load related data. In a data model with many relations this can lead to a lot of unnecessary SQL SELECT's against the database. To postpone loading data until you actually need it (when you fetch it from your Collection with e.g. iterator.next()) you may use the "lazy loading" mechanism. It's simple to use: add the attribute lazy="true" to the field with the Collection, for example on the relation from Actor to Movie:

<field name="movies" 
  type="hansen.playground.Movie" 
  collection="collection" 
  lazy="true">

Let's run the test program again with lazy loading:

***Actor.setMovies: 2 movies to actor "Keanu Reeves"
Movies starring Keanu Reeves
***Media.addMovie: "The Matrix" to media "DVD"
***Movie.addActor: "Keanu Reeves" to movie "The Matrix"
***Actor.setMovies: 1 movies to actor "Laurence Fishburne"
***Movie.addActor: "Laurence Fishburne" to movie "The Matrix"
Movie: The Matrix
  Media: DVD
    Actor: Keanu Reeves
    Actor: Laurence Fishburne
***Media.addMovie: "Chain Reaction" to media "VHS"
***Movie.addActor: "Keanu Reeves" to movie "Chain Reaction"
Movie: Chain Reaction
  Media: VHS
    Actor: Keanu Reeves

- Listing 4: Output from ReadActor with lazy loading enabled -

That's quite a change. The following can be seen:

1. Initially we only created the "Keanu Reeves" instance of Actor, which apparently contained two Movie objects. But as we'll see shortly, these are not yet fully built.
2. The instant we use iterator to fetch a movie Castor will create all the instances needed by the first movie: "The Matrix".
3. When we use iterator for the second and last movie Castor will create the last movie and actor instances.

If we also set lazy loading on the other two Collection fields (Movie to Actor and Media to Movie) we get this output:

***Actor.setMovies: 2 movies to actor "Keanu Reeves"
Movies starring Keanu Reeves
***Media.setMovies: 1 movies to media "DVD"
***Movie.setActors: 2 actors to movie "The Matrix"
Movie: The Matrix
  Media: DVD
    Actor: Keanu Reeves
***Actor.setMovies: 1 movies to actor "Laurence Fishburne"
    Actor: Laurence Fishburne
***Media.setMovies: 1 movies to media "VHS"
***Movie.setActors: 1 actors to movie "Chain Reaction"
Movie: Chain Reaction
  Media: VHS
    Actor: Keanu Reeves

- Listing 5: Output from ReadActor with more lazy loading enabled -

This time it's even more obvious that the objects are created as late as possible.

If you're working with a large database with many relations you will certainly need lazy loading. When you use it you must ensure that you're working inside a transaction (begin/commit) when the objects are lazy loaded. If not you'll get a runtime error like this:

RuntimeException: Transaction is closed!

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