CODEWITHSUNDEEP
grailsgrails-ormgrails-2.0grails-domain-class
Michael
Michael

Reputation: 33297

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

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?

Upvotes: 0

Views: 126

Answers (1)

dmahapatro
dmahapatro

Reputation: 50245

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"
  }
}

Upvotes: 2

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