Ruby on Rails -- SystemStackError: stack level too deep

Question

I added some after create do and before update do code to my model and now I cant save my posts btw both models(user & posts) have a has_and_belongs_to_many relationship:

after_create do
@post = Post.find(self.id)

       for user in  @post.managers.split(',')
     @user = User.find_by_email(user)
    @post.user << @user
    @post.save
  end
end


before_update do
   #
 @post = Post.find(self.id)
       for user in  @post.managers.split(',')

      @user = User.find_by_email(user)
      if @post.user.include?(user)
          #do nothing
      else
    @post.user << @user
   @post.save

end
 end 

What's causing this and what do I change to get rid of this error?