[LRUG] Objects that are neither classes nor records but something in between

[Ed Jones]

Hi Tim

Are you using Rails for this stuff? Regardless of whether it's a Rails project or not, I like using Pundit for problems like this - it abstracts away the authorisation from the model quite nicely. https://github.com/varvet/pundit <https://github.com/varvet/pundit>

So in your case you might have a BookPolicy with an access?(user, book) method, which might look like this:

class BookPolicy
	attr_reader :user, :book

	def initialize(user, book)
		@user = user
		@book = book
	end

	def access?
		user.has_subscription? || user.purchases.includes?(book)
	end
end

Then wherever you need to check if a user can access a book, you just do BookPolicy.new(user, book).access? and your logic to determine that is encapsulated away nicely in the policy class. (Or if you're doing it in Rails controllers, there are helper methods).

This example is predicated on having some sort of way to determine if the user has a subscription or not; (assuming Rails) you could have a polymorphic association to different types of purchase to address this.

Not sure if this is helpful or not!

Ed

> On 22 Jun 2021, at 15:43, Tim Cowlishaw <tim at timcowlishaw.co.uk> wrote:
> 
> Hi there folks! Hope you're all doing well.
> 
> I've just run into a problem / conundrum / anti-pattern / code smell that seems to recur fairly frequently in projects I've worked on of late, and which i'm not really sure how to name in order to search for an existing solution, so i figured I'd post it here in case anyone has some sensible advice, or in the hope that it provokes some interesting discussion, at least.
> 
> The problem basically arises when there's some 'object' in my system (using this word in the vaguest possible sense for now) which has both *data* (so fields that can be updated through an API or webapp, and which should probably be stored in a database row), but also has a one-to-one correspondence with a specific bit of behaviour, expressed as code.
> 
> An example:
> 
> I'm currently working on  a project which is a digital library. It contains lots of Books:
> 
> class Book < Struct.new(:id, :title, :author, :content); end
> 
> Users can buy access to these books either individually (all books are the same price), or by buying a subscription to the whole library. Forgetting the DB / ORM side of this for a sec, i'd model a toy implementation of what i want to do something like the following:
> 
> class User
>   def initialize(purchases=[])
>     @purchases = purchases
>    end
>    attr_reader :purchases
> 
>    def has_access_to?(book)
>      purchases.any? { |p| p.grants_access_to?(book) }
>    end
> end
> 
> class Purchase < Struct.new(:product)
>   def grants_access_to?(book)
>     product.grants_access_to?(book)
>   end 
> end
> 
> class Product
>   def self.price
>    raise NotImplementedError
>   end
> 
>   def self.name <http://self.name/>
>     raise NotImplementedError
>   end
> 
>   def grants_access_to?(book)
>     raise NotImplementedError
>   end
> end
> 
> class OneOffPurchase < Purchase
>   def initialize(book)
>     @book = book
>   end
>   attr_reader :book
> 
>   def self.price
>     500
>   end
> 
>   def self.name <http://self.name/>
>     "A single book"
>   end
> 
>   def grants_access_to?(other_book)
>     book.id <http://book.id/> == other_book.id <http://other_book.id/>
>   end
> end
> 
> def Subscription < Purchase
>   def self.price
>     500
>   end
> 
>   def self.name <http://self.name/>
>     "All inclusive special subscription"
>   end
> 
>   def grants_access_to?(book)
>     true
>   end
> end
> 
> This all to me looks reasonably sensible so far, but i also want those product names and prices to be editable by site admins through our 'backoffice' webapp, so they need to be stored in the DB, and this is where I have trouble finding a solution i'm happy with. Naively I could have an ActiveRecord type object and a fairly simple dispatch mechanism to the code that works out the permissions, but this has some fairly obvious flaws:
> 
> class Product < Struct.new(:unique_key, :name, :price, :book)
>   def grants_accesss_to?(other_book)
>      if unique_key = :one_off && book.present?
>        book.id <http://book.id/> == other_book.id <http://other_book.id/>
>    elsif unique_key == :one_off
>       raise "oops! we're in a totally inconsistent state
>     elsif unique_key == :subscription && book.present?
>        raise "yep, this makes absolutely no sense either"
>     elsif unique_key == :subscription
>       true
>    else
>      raise "this isn't even a real product type. hopeless."
>     end
> end
> 
> aside from all the brittleness and smells that you can see above, this is also kinda useless in a bunch of respects - we can't add new product types without making a change to the codebase, we have to do a bunch of convoluted validation to avoid all the various possible inconsistent states we can get into above, and our codebase is tightly coupled to the value of that unique_key database field, which i'm very suspicious of?
> 
> Therefore, anyone got any smart ideas about how to do this better? The requirements I'm looking to fulfil, in summary:
> 
> 1) A  product has a specific 'strategy' for granting access to a book for a user, expressed as  ruby code
> 2) A product has a price and name that is editable as data through our web app's admin interface
> 
> And, in general, i'd be interested to know -  is there a name for this type of (anti-)pattern - Types of things that refuse to sensibly sit in the domain of software classes or Database fields? Does any of this even make any sense? I'm not so sure myself anymore.
> 
> Any thoughts gratefully received!
> 
> Cheers,
> 
> Tim
> 
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