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problems in software design. Well, it turns out that when working in an object-oriented environment, the problem of how to effectively communicate with a database (which is not object-oriented) is quite common. Therefore, many smart people have wrapped their heads around the problem of how best to bring the object-oriented paradigm together with the relational database. One of those smart people is Martin Fowler, who in his book, Patterns of Enterprise Application Architecture (Addison-Wesley, 2002), first described a pattern that he called an Active Record. In the pattern Fowler described, there is a one-to-one mapping between a database record and the object that represents it. When Rails creator David Heinemeier Hansson sought to implement an ORM for his framework, he based it on Fowler s pattern.
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Active Record lets you model real-world things in your code. Rails calls these realworld things models the M in MVC. A model might be named Person, Product, or Event, and would have a corresponding table in the database: people, products, or events. Each model is implemented as a Ruby class and is stored in the app/models directory. Active Record provides the link between these classes and your tables, allowing you to work with what look like regular objects, which, in turn, can be persisted to the database. This frees you from having to write low-level SQL to talk to the database. Instead, you work with your data as if it were an object, and Active Record does all the translation into SQL behind the scenes. This means that in Rails, you get to stick with one language: Ruby.
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CHAPTER 4 WORKING WITH A DATABASE: ACTIVE RECORD
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Note Just because we re using Active Record to abstract our SQL generation doesn t mean that SQL is
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evil. In fact, Active Record makes it possible to execute SQL directly whenever that s necessary. The truth is that raw SQL is the native language of databases and there are some (albeit rare) cases when an ORM simply won t cut it.
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Active Record Conventions
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Active Record achieves its zero-configuration reputation by way of convention. Most of the conventions it uses are easy to grasp. After all, they re conventions, so they re already in wide use. While you can override most of the conventions to suit the particular design of your database, you ll save a lot of time and energy if you stick to them. Let s take a quick look at the two main conventions you need to know: Class names are singular; table names are plural. Tables contain an identity column named id. Active Record assumes that the name of your table is the plural form of the class name. If your table name contains underscores, then your class name is assumed to be in CamelCase. Table 4-1 shows some examples.
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Table 4-1. Table and Class Name Conventions Table
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events people categories order_items
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Class
Event Person Category OrderItem
All tables are assumed to have a unique identity column named id. This column should be the table s primary key (a value used to uniquely identify a table s row). This is a fairly common convention in database design. (For more on primary keys in database design, the Wikipedia entry has a wealth of useful information and links: http:// en.wikipedia.org/wiki/Primary_key.) The belief in convention over configuration is firmly entrenched in the Rails philosophy, so it should come as no surprise that there are more conventions at work than those listed here. You ll likely find that they all make good sense, and you ll be able to use them without paying much attention.
CHAPTER 4 WORKING WITH A DATABASE: ACTIVE RECORD
Introducing the Console
Ruby comes with a great little tool: an interactive interpreter. It s called irb (for Interactive Ruby). Most of the time, you ll invoke irb using the console program that ships with Rails, but you can start up an irb session whenever you want by typing irb at the command prompt. The advantage of the console is that it enjoys the special privilege of being integrated with your project s environment. This means it has access and knowledge of your models (and subsequently, your database). We ll be using the console as a means to get inside the world of our Event model and to work with it in the exact same way as our Rails application would. As you ll see in a minute, this is a great way to showcase the capabilities of Active Record interactively. You can execute any arbitrary Ruby code in irb and do anything you might otherwise do inside your Ruby programs: set variables, evaluate conditions, and inspect objects. The only essential difference between an interactive session and a regular old Ruby program is that irb will echo the return value of everything it executes. This saves you from having to explicitly print the results of an evaluation. Just run the code, and irb will print the result. You ll be able to tell whenever you re inside an irb session by looking for the double greater-than signs (>>), which indicate the irb prompt, and the arrow symbol (=>), which indicates the response. As you continue to progress with both Ruby and Rails, you ll find that irb is an essential tool. Using irb, you can play around with code and make sure it works as you expect before you write it into your programs. Let s load it and start to experiment with our Event model. Make sure you re inside the event manager application directory, and then type ./script/console on your command line. This will cause the irb console to load with the development environment and leave you at a simple prompt, waiting for you to enter some code.
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