hasMany (relatedMapper, opts) {
    return hasMany(relatedMapper, opts)(this)
  }
Defines a hasMany relationship. Only useful if you're managing your
Mappers manually and not using a Container or DataStore component.

@example
UserMapper.hasMany(PostMapper, {
  // post.user_id points to user.id
  foreignKey: 'user_id'
  // post records will be attached to user records at "user.posts"
  localField: 'posts'
});

@method Mapper#hasMany
@see http://www.js-data.io/v3.0/docs/relations
@since 3.0.0