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

[Andrew Donovan]

Hi Tim,
Your db or permanent storage is your source of truth , if you’re using Rails then use all the facilities it offers such as ActiveRecord. If Product is somehow defining prices and other behaviour for your digital items then it needs to be modelled in the db. If you don’t have a db schema I’d draw out a simply entity relationship diagram which should help in identifying where things are missing or don’t tie up. From the gist of the details below it sounds like you have access to a database. I’d be wary of using code for defining product strategy, amongst other things how do you record product changes over time ?
Regards
Andrew



Regards
Andrew

From: Chat <chat-bounces at lists.lrug.org> On Behalf Of Tim Cowlishaw
Sent: 22 June 2021 15:43
To: Ruby Group <chat at lists.lrug.org>
Subject: [LRUG] Objects that are neither classes nor records b.ut something in between

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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