Rails: Up and Running, 2nd Edition

Attributes

We use the term metaprogramming often in this book, and now you’ve seen it in action. In this case, metaprogramming means that Active Record adds attributes and methods to your code based on the structure of the database without you writing any code. In the end, you’ll code far less, but you’ll have to work a little harder to understand the magic that goes on under the covers. Once you understand how Active Record works, you will know exactly what attributes and methods your model supports. And that’s the secret to using Active Record.

Columns

Let’s review what happens when Ruby loads the Photo class. The Photo class inherits from the ActiveRecord::Base class. When that class loads from the class name Photo, Active Record infers that the class wraps a database table named photos. Base then queries SQLite to retrieve information about the photos table, including the definition of each of the columns in that table. Next, Base saves the definition of each column into the @@columns class variable. (A class variable has a single value that all instances of the class share.) @@columns is an array of Column objects, with each column having these attributes:

name

The name of the database column.

type

The Ruby type of the attribute this column will generate.

number

A Boolean value that’s true if the column’s data is numeric. You’ll access it through the accessor method number?.

limit

The maximum size of the data element. For example, for a database column of type varchar(45) ...