console_tests.md

[1] pry(#)> self.member = Array.new

=> []

[2] pry(#)> self.member.push( User.find(memberUid.first).dn )

NoMethodError: You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.push

The code in the ruby file is a little different. The above resulted by removing the each loop for debugging.

[1] pry(#<Group>)> puts "#{self.member.class}" 
Array
=> nil
[2] pry(#<Group>)> self.member = []
=> []
[3] pry(#<Group>)> puts "#{self.member.class}"
NilClass
=> nil

original_file.rb

class Group < ActiveLdap::Base
  ldap_mapping :dn_attribute => "cn",
               :prefix => "ou=Groups",
               :classes => ["top",
                            "posixGroup",
                            "groupOfNames"
                            ]

    has_many :users,
             :wrap => "memberUid",
             :class_name => "User",
             :primary_key => 'uid'

    before_save :set_members_array

    def set_members_array
      # TODO MKL fix this
      require 'pry'
      binding.pry 
      self.member = []
      memberUid.each{ |uid|
        self.member.push( User.find(uid).dn )
      }
    end

    def as_json(options = {})
      self.attributes
    end

end

solution.md

The problem here was, that when setting an ActiveLdap attribute to an empty array (Array.new or just []) it will set the actual attribute to nil. You can avoid this by never setting an attribute to be an empty array. Means using an extra variable or something like this

self.member = memberUid.collect{|uid| User.find(uid).dn}