How to implement Self-Referencing Relationships in Grails? - grails

Question

Given the following User class:
class User {
String name
static hasMany = [friends: User]
}
I want that a User can have many friends which are instances of the user domain class.
How do I have to implement the friend relationship of a user?

Answer 1

1. How Do you Define the relathionship
class User {
static hasMany = [ friends: User ]
static mappedBy = [ friends: 'friends' ] //this how you refer to it using GORM as well as Database
String name
String toString() {
name
}
def static constrains () {
name(nullable:false,required:true)
}
def static mapping={
/ / further database custom mappings ,like custom ID field/generation
}
}
2.How to save Data:
def init = {servletContext->
if(User?.list()==null) { // you need to import User class :)
def user = new User(name:"danielad")
def friends= new User(name:'confile')
def friends2=new User(name:'stackoverflow.com')
user.addToFriends(friends)
user.addToFriends(friends2)
user.save(flash:true)
}
}
3# . Your question is repeated on this stack overflow link :
Maintaining both sides of self-referential many-to-many relationship in Grails domain object

Answer 2

It looks like many-to-many relationship (one user has a lot of friends, and is a friend of a lot of users). So one of the solution will be to create new domain class, lets say it Frendship. And then modify User domain class like here:
class Friendship {
belongsTo = [
friend1: User
, friend2: User
]
}
class User{
String name
hasMany = [
hasFriends: Friendship
, isFriendOf: Friendship
]
static mappedBy = [
hasFriends: 'friend1'
, isFriendOf: 'frined2'
]
}