Basically, the has_many association controls how the "join model" (Subscription) is created and the has_many_through association controls how the :through model is created. So instead of having one has_many :subscriptions I split it to two...

gistfile1.rb

  has_many  :subscriptions_members, 
            :class_name => "Subscription",
            :conditions => {:kind => Subscription::SubscriptionKinds::MEMBER},
            :dependent => :destroy

  has_many  :subscriptions_followers, 
            :class_name => "Subscription",
            :conditions => {:kind => Subscription::SubscriptionKinds::FOLLOWER},
            :dependent => :destroy
  
  has_many :members,  :through => :subscriptions_members, 
                      :source => :user, :uniq => true
    
  has_many :followers,:through => :subscriptions_followers, 
                      :source => :user, :uniq => true

And this sets the :kind field by itself?

Yes. the :conditions also control the object creation.
So subscriptions_members controls how the Subscription instance is created and members how a user will be created

Nice!