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CHAPTER 4 WORKING WITH A DATABASE: ACTIVE RECORD
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Introducing Active Record: Object-Relational Mapping on Rails
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The key feature of Active Record is that it maps tables to classes, table rows to objects, and table columns to object attributes. This practice is commonly known as objectrelational mapping, or ORM for short. To be sure, Active Record isn t the only ORM in existence, but it may well be the easiest to use of the bunch. One of the reasons that Active Record is so easy to use is that there is almost no configuration required to have it map a table to a class. You just need to create a Ruby class that s named after the table you want to map and extend the Active Record Base class.
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class Event < ActiveRecord::Base end
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Notice the part that reads < ActiveRecord::Base. That less-than sign indicates that the class on the left is a subclass of the one on the right. In Ruby, when you extend a class like this, you automatically gain access to all the functionality in the parent class. There s a lot of code in the ActiveRecord::Base class, but we don t need to look at it. Our class merely extends it, and our work is finished. Assuming Active Record knows how to find our database and that we have a table called events (note that the table name is plural while the class name is singular), the table is automatically mapped. If our events table contained the fields title, location, and occurs_on, we could do this:
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event = Event.new event.title = "BBQ at Hampton's House" event.location = "471 230 King Street" event.occurs_on = "2006-10-13" event.save
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Those five lines would write a new record to the events table. That s a lot of ability we gained just by the simple act of subclassing! And that s what we mean when we say that Active Record is easy to use. Notice how the table s fields (title, location, and occurs_on) can be read and written to using methods on the object we created (event). And we didn t need to tell Active Record what our fields were named, or even that we had any fields. It figured that out all on its own. Of course, Active Record doesn t just let you create new records. It can also read, update, and delete records, plus a lot more. Active Record is database-agnostic, so it doesn t care which database software you use, and it supports nearly every database out there. Since it s a high-level abstraction, the code you write remains the same no matter which database you re using. For the
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CHAPTER 4 WORKING WITH A DATABASE: ACTIVE RECORD
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record (no pun intended), we ll be using MySQL. As explained in 2, MySQL is open source, easy to use, fast, and arguably the most popular database used for Rails development. (Along with the MySQL site, http://mysql.com, the Wikipedia entry on MySQL is an excellent resource: http://en.wikipedia.org/wiki/Mysql.)
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What About SQL
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To be sure, you don t need Active Record (or any ORM) to talk to and manipulate your database. In fact, databases have their own language called Structured Query Language, popularly referred to as SQL, which is supported by nearly every database in existence. Using SQL, you can view column information, fetch a particular row or a set of rows, and search for rows containing certain criteria. You also use SQL to create, drop, and modify tables and insert, update, and destroy the information stored in those tables. The problem with SQL is that it s not object-oriented. Object-oriented programming and relational databases are fundamentally different paradigms. The relational paradigm deals with relations and is mathematical by nature. The object-oriented paradigm, however, deals with objects, their attributes, and their associations to each other. As soon as you want to make objects persistent using a relational database, you will notice something: there is a rift between these two paradigms the so-called object-relational gap. An ORM library like Active Record helps you bridge that gap.
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