Automatically create associations through many existing associations - ruby-on-rails

Question

I'm working on an engine where any model can have a has_many association with Permit as Permissible:
class Permit < ActiveRecord::Base
belongs_to :permissible, polymorphic: true
end
module Permissible
def self.included(base)
base.class_eval do
has_many :permits, as: :permissible
end
end
class Group < ActiveRecord::Base
include Permissible
end
class GroupAllocation < ActiveRecord::Base
belongs_to :person
belongs_to :group
end
class Person < ActiveRecord::Base
include Permissible
has_many :group_allocations
has_many :groups, through: :group_allocations
end
class User < ActiveRecord::Base
belongs_to :person
end
So, Group has_many :permits and Person has_many :permits. What I am trying to do is dynamically create associations on User that uses the permits association as a source, and chain associations on other models down to User by doing the same. This can be done manually (in rails 3.1+) with:
class Person
has_many :group_permits, through: :person, source: :permits
end
class User
has_many :person_permits, through: :person, source: :permits, class_name: Permit
has_many :person_group_permits, through: :person, source: :group_permits, class_name: Permit
end
However, in practice, Permissible will be included on many models, so I'm trying to write a class method on User (actually within another module, but no need to confuse things more) that can traverse User.reflect_on_all_associations and create an array of new associations, which may be many associations deep each.
Looking for input on how to do this cleanly in rails 3.2.8.

Answer 1

Here is how I did it (implementation code varies slightly from the details given in the question):
module Authorisable
def self.included(base)
base.class_eval do
base.extend ClassMethods
end
end
module ClassMethods
class PermissionAssociationBuilder
def build_permissions_associations(klass)
chains = build_chains_from(klass)
chains.select! {|c| c.last.klass.included_modules.include? DistributedAuthorisation::Permissible}
permissions_associations = []
chains.each do |chain|
source_name = :permissions
chain.reverse.each do |r|
assoc_name = :"#{r.name}_#{source_name}"
r.active_record.has_many assoc_name, through: r.name.to_sym, source: source_name, class_name: DistributedAuthorisation::Permission
source_name = assoc_name
end
permissions_associations << source_name
end
return permissions_associations
end
private
def build_chains_from(klass)
chains = reflections_to_follow(klass).map {|r| [r]}
chains.each do |chain|
models = chain.map {|r| r.klass}.unshift klass
reflections_to_follow(models.last).each do |r|
chains << (chain.clone << r) unless models.include? r.klass
end
end
end
def reflections_to_follow(klass)
refs = klass.reflect_on_all_associations
refs.reject {|r| r.options[:polymorphic] or r.is_a? ActiveRecord::Reflection::ThroughReflection}
end
end
def permissions_associations
#permissions_associations ||= PermissionAssociationBuilder.new.build_permissions_associations(self)
end
end
Probably not the most efficient method, but it adds the chains I'm after with Klass.permissions_associations, and stores their symbols in class instance variable.
I'd be happy to hear suggestions on how to improve it.