How to limit the Relation Elements of a Class in Grails?

Question

I have

class User {
  String name

  hasMany = [books: Book]
}


class Book (
  String name
  belongsTo = [user: User]
}

Now, I can access all books of a user instance as:

def user = User.find("someId")
println user.books

How can I limit the number of books such that I get only the first x books from user.books? Is there also a way to sort them?

Answer 1

One viable approach is to define books as List inside User. You would need to have an index column but you could get the benefit of pagination and ordering like:

class User{
    List books
    static hasMany = [books: Book]
}

def user = User.find("someId")
println user.books?.getAt(3..10).sort{it.name}

Note:-
All books for User will be fetched lazily, using the above approach will filter books from index 3 till 10.

In case you want to optimize the lazy fetch strategy (N + 1), then you would probably need to have a look at batchSize and order. Also sort as a side note.

Example:

class User {
  String name
  static hasMany = [books: Book]
  static mapping = {
      books batchSize: 10
  }
}

class Book (
  String name
  static belongsTo = [user: User]
  static mapping = {
      order "desc"
  }
}