Add a Why not? section

README.md

@@ -21,6 +21,7 @@ Django styleguide that we use in [HackSoft](https://hacksoft.io).
 <!-- toc -->
 
 - [Overview](#overview)
+- [Why not?](#why-not)
 - [Cookie Cutter](#cookie-cutter)
 - [Models](#models)
   * [Base model](#base-model)
@@ -77,12 +78,14 @@ Django styleguide that we use in [HackSoft](https://hacksoft.io).
 
 ## Overview
 
+The core of the Django Styleguide can be summarized as follows:
+
 **In Django, business logic should live in:**
 
-* Model properties (with some exceptions).
-* Model `clean` method for additional validations (with some exceptions).
 * Services - functions, that mostly take care of writing things to the database.
 * Selectors - functions, that mostly take care of fetching things from the database.
+* Model properties (with some exceptions).
+* Model `clean` method for additional validations (with some exceptions).
 
 **In Django, business logic should not live in:**
 
@@ -90,12 +93,69 @@ Django styleguide that we use in [HackSoft](https://hacksoft.io).
 * Serializers and Forms.
 * Form tags.
 * Model `save` method.
+* Custom managers or querysets.
+* Signals.
 
 **Model properties vs selectors:**
 
 * If the property spans multiple relations, it should better be a selector.
 * If the property is non-trivial & can easily cause `N + 1` queries problem, when serialized, it should better be a selector.
 
+The general idea is to "separate concerns" so those concerns can be maintainable / testable.
+
+## Why not?
+
+> Why not put your business logic in APIs / Views / Serializers / Forms?
+
+Relying on generic APIs / Views, with the combination of serializers & forms does 2 major things:
+
+1. Fragments the business logic in multiple places, making it really hard to trace the data flow.
+2. Hides things from you. In order to change something, you need to know the inner-workings of the abstraction that you are using.
+
+Generic APIs & Views, in combination with serializers & forms is really great for the straight-forward "CRUD over a model" case.
+
+From our experience so far, this straightforwad case rarely happens. And once you leave the happy CRUD path, things start to get messy.
+
+And once things start to get messy, you need more "boxes", to organize your code in a better way.
+
+This styleguide aims to:
+
+1. Give you those "boxes".
+1. Help you figure out your own "boxes", for your own specific context & needs.
+
+---
+
+> Why not put your business logic in custom managers and/or querysets?
+
+This is actually a good idea & you might introduce custom managers & querysets, that can expose better API, tailored to your domain.
+
+But trying to place all of your business logic in a custom manager is not a great idea, because of the following:
+
+1. Business logic has it's own domain, which is not always directly mapped to your data model (models)
+1. Business logic most often spans across multiple models, so it's really hard to choose where to place something.
+    - Lets say you have a custom piece of logic that touches models `A`, `B`, `C` and `D`. Where do you put it?
+1. There can be additional calls to 3rd party systems. You don't want those in your custom manager methods.
+
+**The idea is to let your domain live separately, from your data model & API layer.**
+
+If we take the idea for having custom managers and combine that with the idea of letting the domain live saparetely, we'll end up with what we call a "service layer".
+
+Services can be functions, classes, modules or whatever makes sense for your particular case.
+
+With all that in mind, custom managers & querysets are very powerful tool and should be used.
+
+---
+
+> Why not put your business logic in signals?
+
+From all of the available options, perhaps, this one will lead you to a very bad place very quickly:
+
+1. Signals are a great tool for **connecting things that should not know about each other, yet, you want them to be connected.**
+1. Signals are also a great tool **for handling cache invalidation** outside your business logic layer.
+1. If we start using signals for things that are heavily connected, we are just making the connection more implicit and making it harder to trace the data flow.
+
+That's why we recommend using signals for very particular use cases, but generally, **we don't recommend using them for structuring the domain / business layer.**
+
 ## Cookie Cutter
 
 We recommend starting every new project with some kind of cookiecutter. Having the proper structure from the start pays off.