many-to-many in grails is not saving records in the relationship table

Question

I've got a many-to-many association between Color and Shade. Color has many shades and Shades has many colors.

I've modeled this like so:

class Color {
  static hasMany = [shades: Shade]
  String name
}

class Shade {
  static belongsTo = Color
  static hasMany = [colors: Color]
  String name
}

However, when I run the following code:

new Color(name: "Red").addToShades(new Shade(name: "light")).save()

It only saves record in Color table and Shade table but not in Color_Shades table which is essentially a join table between the two.

Am I doing something wrong? Thats how I understood it from the docs:

Answer 1

I'm not sure why your table isn't populated, but there's an advise of Burt in this talk about the performance using this type of many-to-many. The solution is to use an intermediate class:

class ColorShade implements Serializable {

  Color color

  Shade shade

  //implement hashcode & equals!
  //and also implement helpers like removeAll, remove, create and get.

  static mapping = {
    id composite: ['color','shade']
    table 'Color_Shades'
    version false
  }
}

You can see a example class in the Spring Security Core plugin.