Batcan VS CanCan

gistfile1.rb

# Batcan Style
class Project
  # permission is enforced for all create/update/save calls. It is defined within the context of its own class and is only
  # called when this particular permission is checked.
	permission :save do |project, user|
		next true if user.admin?
		next true if project.owner == user
		next true if project.collaborators.include?(user) 
		# at this point if we haven't returned true then we dont have permission. We are allowed to return a user readable
		# string here (not sure if CanCan has this capability, I don't see it in the docs)
		"Only admins, project owners and project collaborators can save this project"
	end
	
  # only enforced on create/update/save if due_date is modified
  # NOTE: I don't believe CanCan has this capability, but I may be wrong
	permission :save, :due_date do |project, user|
		next true if user.admin? or project.owner == user
		"Only admins and project owners can set the due date for this project"
	end
end


# CanCan Style
class Project
end

# permissions for all objects are defined within one large initialize script within the ability class. 
# Imagine if there were 50 permissions defined within the system. Note that the collaborators check here would make a database
# call. What if we don't need to check this permission? We end up making the call for no reason.
class Ability
  include CanCan::Ability
  
  def initialize(user)
    alias_action :create, :update, :to => :save
    can :save Project do |project|
     user.admin? || project.owner == user || project.collaborators.include?(user)    
    end
  end  
end


### Usage:
project = Project.find('some-project')

## BatCan:

# can be called from anywhere in the code. Models, Specs, Rake Tasks... anywhere.
user.can? :save, project

## CanCan:

# can only be called from a controller.
can? :save, project

I think there might be some confusion on the CanCan usage because it can actually be used in Views and Helpers as well: https://github.com/ryanb/cancan/wiki/Link-Helpers.
Also on a different model than the current_user as in your line 48 usage: https://github.com/ryanb/cancan/wiki/Ability-for-Other-Users