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The MVC Pattern
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Rails employs a time-honored and well-established architectural pattern that advocates a division of application logic and labor into three distinct categories: the model, view, and controller. In the MVC pattern, the model represents the data, the view represents the user interface, and the controller directs all the action. The real power lies in the combination of the MVC layers, which is something that Rails handles for you. Place your code in the right place and follow the naming conventions, and everything should fall into place.
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CHAPTER 1 INTRODUCING THE RAILS FRAMEWORK
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Each part of the MVC the model, view, and controller is a separate entity, capable of being engineered and tested in isolation. A change to a model need not affect the views; likewise, a change to a view should have no effect on the model. This means that changes in an MVC application tend to be localized and low impact, easing the pain of maintenance considerably, while increasing the level of reusability among components. Contrast this to the situation that occurs in a highly coupled application that mixes data access, business logic, and presentation code (PHP, we re looking at you). Some folks call this spaghetti code because of its striking resemblance to a tangled mess. In such systems, duplication is common, and even small changes can produce large ripple effects. MVC was designed to help solve this problem. MVC isn t the only design pattern for web applications, but it does happen to be the one that Rails has chosen to implement. And it turns out that it works great for web development. By separating concerns into different layers, changes to one of them don t have an impact on the others, resulting in faster development cycles and easier maintenance.
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The MVC Cycle
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Although MVC comes in different flavors, control flow generally works as follows (see Figure 1-1): 1. The user interacts with the interface and triggers an event (for example, submits a registration form). 2. The controller receives the input from the interface (for example, the submitted form data). 3. The controller accesses the model, often updating it in some way (for example, by creating a new user with the form data). 4. The controller invokes a view that renders an updated interface (for example, a welcome screen). 5. The interface waits for further interaction from the user, and the cycle repeats.
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CHAPTER 1 INTRODUCING THE RAILS FRAMEWORK
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Figure 1-1. The MVC cycle
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If the whole MVC concept sounds a little involved at first, don t worry. While entire books could be written on this pattern, and people will argue over its purest implementation for all time, you ll find that it s really quite easy to grasp, especially the way Rails does MVC. Next, we ll take a quick tour through each letter in the MVC, and then describe how Rails handles it.
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The Layers of MVC
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The three layers of the MVC pattern work together as follows: Model: The information the application works with. View: The visual representation of the user interface. Controller: The director of interaction between the model and the view.
Models
In Rails, the model layer represents the database. While we call the entire layer the model, Rails applications are usually made up of several individual models, each of which (usually) maps to a database table. For example, a model called User would map to a table called users. The User model assumes responsibility for all access to the users table in the database, including creating, reading, updating, and deleting rows. So, if you wanted to work with the table and, say, search for someone by name, you would do so through the model, like this:
User.find_by_name('Linus')
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CHAPTER 1 INTRODUCING THE RAILS FRAMEWORK
This snippet, while very basic, would search the users table for the first row with the value Linus in the name column and return the results. To achieve this, Rails uses its builtin database abstraction layer, Active Record. Active Record is a powerful library, so needless to say, this is only a small portion of what you can do with it. s 4 and 5 will give you an in-depth understanding of Active Record and what you can expect from it. For the time being, the important thing to remember is that models represent data. All rules for data access, associations, validations, calculations, and routines that should be executed before and after save, update, or destroy operations are neatly encapsulated in the model. Your application s world is populated with Active Record objects: single ones, lists of them, new ones, and old ones. And Active Record lets you use Ruby language constructs to manipulate all of them, meaning you get to stick to one language for your entire application.
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