Why grails throwing null pointer exception while accessing hasMany relationship first time?

Question

I have a strange problem.
I have two domain classes User and Post with fields:

class User {
  String name
  static hasMany = [pos         


        

Answer 1

You can prevent this by explicitly declaring your collection property (with a value) alongside your mapping:

class User {
    String name
    Set posts = []
    static hasMany = [posts: Post]
}

You can define the collection type you need. The default is Set, but if you need to maintain order, you might consider List or SortedSet.