On Unit Tests and Layers

This weekend I had an idea on why some people insist on writing tests for every single function and object, but first you must know that 3 things happened:

First, I told someone a programmer joke like this: "You can't teach top-down development to newcomers 'cause they don't know which side is up." Yes, it is a joke; yes, it is somewhat fun, but at the same time, it is a reality: we should let newcomers get wet before telling them how to properly design their project.

The second is that I got "drafted" to do a tutorial on DjangoRestFramework and, because I was expecting someone to ask how to manage validation on forms and serializers [1] and decided to learn a bit more about "fat models", which basically says you should put all your business rules on the model layer. And here is the part that stuck with me: the model is not an object, but a layer.

The third is that I was summoned to explain to a group to explain what should be tested. Thing is, they did a "fizzbuzz" code in which they had multiple_of_3, multiple_of_5, multiple_of_15 and someone mentioned that we should test them too.

And that's what got me thinking: Should we test it? My response for this question is always "no, think your project is a black box in which you press a button and some light goes on; you don't need to know if there is a nerve somewhere in your knee that you hit it and your leg does some kicking or if the signal is sent all the way to your brain and it sends a response back to the muscle to make it kick, all you need to know -- and test -- is that when you hit someone in the knee, they kick and that's your requirement and that's what you should test." But what if it is not that simple?

The model layer has a responsability and has its own requirements: It should receive the data and store it somewhere; it can expose some business rules (as validation of said data) but shouldn't run those rules. The controller layer also has its own responsabilities and its own requireements: it gets the data from somewhere, do the checks, validate the business rules and then sends it to the model layer to be store. The view layer, again, has its own responsabilities and its own requirements: exposes the data to the user and receives data from the user and passes it to the controller layer, exposing the errors back to the user.

In a way, all those layers could be separate projects: because they have a defined API, one could replace each layer independently -- I could have a model layer that stores all that in JSON and then replace it to one that stores in a Stream Processing Database and the controller and view layers will never know about. All those layers could be separate libraries -- a lost art of getting code with a single functionality and giving it its own space, with its own build tools and a well defined interface and which is its used by other projects as a black box: they know the interface, but they don't know how, internally, the library does whatever it does.

And, because each library has its own code, it will have its own tests.

And that's what people were seeing: They were seeing, unconciously, "this is another layer in the project, this layer should be tested because it can be replaced any time". Each multilple_of function was an API exposed from the model layer and, thus, should be tested. Even if the design doesn't seem to be layered and appear all around the code.

Thing is, each layer is nothing by itself: There is no use for exposing business/validation rules without a controller to send the data; there is no use for something to validate incoming data and apply business rules if there is noone sending the data and noone to store it; there is no use for something responsibile for calling business rules check if there isn't business rules. All three layers are required to do the word -- and that's probably why I believe you should test them all as a single unity.

But, in the very end, it seems that going full force into unit tests and what should be tested can actually help people see they layers of their projects.

[1]"forms" are responsible for validating incoming HTML form content; "serializers" are responsible for validating incoming data from the API -- although "serializers" are also responsible for serializing the output data back to the client.
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