<div dir="ltr"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">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? </blockquote><div><br></div>What you describe sounds a lot like an <a href="https://martinfowler.com/bliki/AnemicDomainModel.html">anemic domain model</a>.<div><br></div><div>From Martin Fowler's article:</div><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">The fundamental horror of this anti-pattern is that it's so contrary to the basic idea of object-oriented design; which is to combine data and process together. The anemic domain model is really just a procedural style design, exactly the kind of thing that object bigots like me (and Eric) have been fighting since our early days in Smalltalk. What's worse, many people think that anemic objects are real objects, and thus completely miss the point of what object-oriented design is all about.</blockquote><div><br></div><div>-- Alberto </div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, Jun 22, 2021 at 3:48 PM Tim Cowlishaw <<a href="mailto:tim@timcowlishaw.co.uk">tim@timcowlishaw.co.uk</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div>Hi there folks! Hope you're all doing well.</div><div><br></div><div>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.</div><div><br></div><div>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.</div><div><br></div><div>An example:</div><div><br></div><div>I'm currently working on  a project which is a digital library. It contains lots of Books:</div><div><br></div><div style="margin-left:40px"><span style="font-family:monospace">class Book < Struct.new(:id, :title, :author, :content); end</span></div><div><br></div><div>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:<br></div><div><span style="font-family:monospace"><br></span></div><div style="margin-left:40px"><span style="font-family:monospace">class User</span></div><div style="margin-left:40px"><span style="font-family:monospace">  def initialize(purchases=[])</span></div><div style="margin-left:40px"><span style="font-family:monospace">    @purchases = purchases</span></div><div style="margin-left:40px"><span style="font-family:monospace">   end</span></div><div style="margin-left:40px"><span style="font-family:monospace">   attr_reader :purchases</span></div><div style="margin-left:40px"><span style="font-family:monospace"><br></span></div><div style="margin-left:40px"><span style="font-family:monospace">   def has_access_to?(book)</span></div><div style="margin-left:40px"><span style="font-family:monospace">     purchases.any? { |p| p.grants_access_to?(book) }<br></span></div><div style="margin-left:40px"><span style="font-family:monospace">   end<br></span></div><div style="margin-left:40px"><span style="font-family:monospace">end</span><br></div><div style="margin-left:40px"><br></div><div style="margin-left:40px"><span style="font-family:monospace">class Purchase < Struct.new(:product)</span></div><div style="margin-left:40px"><span style="font-family:monospace">  def grants_access_to?(book)</span></div><div style="margin-left:40px"><span style="font-family:monospace">    product.grants_access_to?(book)<br></span></div><div style="margin-left:40px"><span style="font-family:monospace">  end <br></span></div><div style="margin-left:40px"><span style="font-family:monospace">end</span></div><div style="margin-left:40px"><span style="font-family:monospace"><br></span></div><div style="margin-left:40px"><span style="font-family:monospace">class Product</span></div><div style="margin-left:40px"><span style="font-family:monospace">  def self.price</span></div><div style="margin-left:40px"><span style="font-family:monospace">   raise NotImplementedError<br></span></div><div style="margin-left:40px"><span style="font-family:monospace">  end</span></div><div style="margin-left:40px"><span style="font-family:monospace"><br></span></div><div style="margin-left:40px"><span style="font-family:monospace">  def <a href="http://self.name" target="_blank">self.name</a></span></div><div style="margin-left:40px"><span style="font-family:monospace">    raise NotImplementedError</span></div><div style="margin-left:40px"><span style="font-family:monospace">  end<br></span></div><div style="margin-left:40px"><span style="font-family:monospace"><br></span></div><div style="margin-left:40px"><span style="font-family:monospace">  def grants_access_to?(book)</span></div><div style="margin-left:40px"><span style="font-family:monospace">    raise NotImplementedError<br></span></div><div style="margin-left:40px"><span style="font-family:monospace">  end<br></span></div><div style="margin-left:40px"><span style="font-family:monospace">end</span></div><div style="margin-left:40px"><span style="font-family:monospace"><br></span></div><div style="margin-left:40px"><span style="font-family:monospace">class OneOffPurchase < Purchase</span></div><div style="margin-left:40px"><span style="font-family:monospace">  def initialize(book)</span></div><div style="margin-left:40px"><span style="font-family:monospace">    @book = book</span></div><div style="margin-left:40px"><span style="font-family:monospace">  end</span></div><div style="margin-left:40px"><span style="font-family:monospace">  attr_reader :book</span></div><div style="margin-left:40px"><span style="font-family:monospace"><br></span></div><div style="margin-left:40px"><span style="font-family:monospace">  def self.price</span></div><div style="margin-left:40px"><span style="font-family:monospace">    500</span></div><div style="margin-left:40px"><span style="font-family:monospace">  end</span></div><div style="margin-left:40px"><span style="font-family:monospace"><br></span></div><div style="margin-left:40px"><span style="font-family:monospace">  def <a href="http://self.name" target="_blank">self.name</a></span></div><div style="margin-left:40px"><span style="font-family:monospace">    "A single book"<br></span></div><div style="margin-left:40px"><span style="font-family:monospace">  end</span></div><div style="margin-left:40px"><span style="font-family:monospace"><br></span></div><div style="margin-left:40px"><span style="font-family:monospace">  def grants_access_to?(other_book)</span></div><div style="margin-left:40px"><span style="font-family:monospace">    <a href="http://book.id" target="_blank">book.id</a> == <a href="http://other_book.id" target="_blank">other_book.id</a><br></span></div><div style="margin-left:40px"><span style="font-family:monospace">  end</span></div><div style="margin-left:40px"><span style="font-family:monospace">end</span></div><div style="margin-left:40px"><span style="font-family:monospace"><br></span></div><div style="margin-left:40px"><span style="font-family:monospace">def Subscription < Purchase</span></div><div style="margin-left:40px"><span style="font-family:monospace">  def self.price</span></div><div style="margin-left:40px"><span style="font-family:monospace">    500</span></div><div style="margin-left:40px"><span style="font-family:monospace">  end</span></div><div style="margin-left:40px"><span style="font-family:monospace"><br></span></div><div style="margin-left:40px"><span style="font-family:monospace">  def <a href="http://self.name" target="_blank">self.name</a></span></div><div style="margin-left:40px"><span style="font-family:monospace">    "All inclusive special subscription"</span></div><div style="margin-left:40px"><span style="font-family:monospace">  end</span></div><div style="margin-left:40px"><span style="font-family:monospace"><br></span></div><div style="margin-left:40px"><span style="font-family:monospace">  def grants_access_to?(book)</span></div><div style="margin-left:40px"><span style="font-family:monospace">    true<br></span></div><div style="margin-left:40px"><span style="font-family:monospace">  end</span></div><div style="margin-left:40px">end</div><div style="margin-left:40px"><br></div><div>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:</div><div><br></div><div style="margin-left:40px"><span style="font-family:monospace">class Product < Struct.new(:unique_key, :name, :price, :book)</span></div><div style="margin-left:40px"><span style="font-family:monospace">  def grants_accesss_to?(other_book)</span></div><div style="margin-left:40px"><span style="font-family:monospace">     if unique_key = :one_off && book.present?<br></span></div><div style="margin-left:40px"><span style="font-family:monospace">       <a href="http://book.id" target="_blank">book.id</a> == <a href="http://other_book.id" target="_blank">other_book.id</a></span></div><div style="margin-left:40px"><span style="font-family:monospace">   elsif unique_key == :one_off</span></div><div style="margin-left:40px"><span style="font-family:monospace">      raise "oops! we're in a totally inconsistent state<br></span></div><div style="margin-left:40px"><span style="font-family:monospace">    elsif unique_key == :subscription && book.present?<br></span></div><div style="margin-left:40px"><span style="font-family:monospace">       raise "yep, this makes absolutely no sense either"</span></div><div style="margin-left:40px"><span style="font-family:monospace">    elsif unique_key == :subscription</span></div><div style="margin-left:40px"><span style="font-family:monospace">      true</span></div><div style="margin-left:40px"><span style="font-family:monospace">   else</span></div><div style="margin-left:40px"><span style="font-family:monospace">     raise "this isn't even a real product type. hopeless."<br></span></div><div style="margin-left:40px"><span style="font-family:monospace">    end<br></span></div><div style="margin-left:40px"><span style="font-family:monospace">end</span></div><div><br></div><div>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?</div><div><br></div><div>Therefore, anyone got any smart ideas about how to do this better? The requirements I'm looking to fulfil, in summary:</div><div><br></div><div>1) A  product has a specific 'strategy' for granting access to a book for a user, expressed as  ruby code</div><div>2) A product has a price and name that is editable as data through our web app's admin interface</div><div><br></div><div>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.<br></div><div><br></div><div>Any thoughts gratefully received!</div><div><br></div><div>Cheers,</div><div><br></div><div>Tim</div><div><br></div></div>
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</blockquote></div>