The MTV Development Pattern

Before we delve into any more code, let’s take a moment to consider the overall design of a database-driven Django Web application. As we mentioned in previous chapters, Django is designed to encourage loose coupling and strict separation between pieces of an application. If you follow this philosophy, it’s easy to make changes to one particular piece of the application without affecting the other pieces. In view functions, for instance, we discussed the importance of separating the business logic from the presentation logic by using a template system. With the database layer, we’re applying that same philosophy to data access logic.

Those three pieces together — data access logic, business logic, and presentation logic — comprise a concept that’s sometimes called the Model-View-Controller (MVC) pattern of software architecture. In this pattern, “Model” refers to the data access layer, “View” refers to the part of the system that selects what to display and how to display it, and “Controller” refers to the part of the system that decides which view to use, depending on user input, accessing the model as needed.

Why the Acronym?  The goal of explicitly defining patterns such as MVC is mostly to streamline communication among developers. Instead of having to tell your coworkers, “Let’s make an abstraction of the data access, then let’s have a separate layer that handles data display, and let’s put a layer in the middle that regulates this,” you can take advantage of a shared vocabulary and say, “Let’s use the MVC pattern here.”

Django follows this MVC pattern closely enough that it can be called an MVC framework. Here’s roughly how the M, V, and C break down in Django:

• M, the data-access portion, is handled by Django’s database layer, which is described in this chapter.
• V, the portion that selects which data to display and how to display it, is handled by views and templates.
• C, the portion that delegates to a view depending on user input, is handled by the framework itself by following your URLconf and calling the appropriate Python function for the given URL.

Because the “C” is handled by the framework itself and most of the excitement in Django happens in models, templates, and views, Django has been referred to as an MTV framework. In the MTV development pattern,

• M stands for “Model,” the data access layer. This layer contains anything and everything about the data: how to access it, how to validate it, which behaviors it has, and the relationships between the data.
• T stands for “Template,” the presentation layer. This layer contains presentation-related decisions: how something should be displayed on a Web page or other type of document.
• V stands for “View,” the business logic layer. This layer contains the logic that access the model and defers to the appropriate template(s). You can think of it as the bridge between models and templates.

If you’re familiar with other MVC Web-development frameworks, such as Ruby on Rails, you may consider Django views to be the “controllers” and Django templates to be the “views.” This is an unfortunate confusion brought about by differing interpretations of MVC. In Django’s interpretation of MVC, the “view” describes the data that gets presented to the user; it’s not necessarily just how the data looks, but which data is presented. In contrast, Ruby on Rails and similar frameworks suggest that the controller’s job includes deciding which data gets presented to the user, whereas the view is strictly how the data looks, not which data is presented.

Neither interpretation is more “correct” than the other. The important thing is to understand the underlying concepts.

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