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A Rails Model is a Ruby class that can add database records (think of whole rows in an Excel table), find particular data you're looking for, update that data, or remove data. These common operations are referred to by the acronym CRUD--Create, Remove, Update, Destroy.
There are other ways to perform CRUD operations on databases--the most common way is to use a Data Definition Language like SQL--but Rails makes the whole business a lot easier and more extensible for reasons described in the entry on migrations.
Since the model can perform CRUD operations on an entire table in the database, it should be given a description of every column in that table. Again, thinking of Excel, if we had a table for a users in our system, we would want to think of each type of data we'd like to keep track of for individual users. In the following example, I've used just a username and password. (We wouldn't actually want to store unencrypted passwords in the database, but we'll leave that concept out for now).
The simplest way to write a model class is to let Rails do it for you. Rails contains a model generator, which you can use via your command line, as long as you're in a Rails app already.
rails generate model ModelName ColumnOneName:ColumnOneType ColumnTwoName:ColumnTwoType
rails generate model User username:string password:string
As we'll see in the command line, the model generator generates a number of files for us:
create db/migrate/20130518173035_create_users.rb create app/models/user.rb create test/unit/user_test.rb create test/fixtures/users.yml
I've left out a few statements to show only the files that have been generated for us.
First is the migration file, which is stored in
db/migratelike all other migrations files. Migrations describe the changes we'll be making to our database (though haven't actually enacted just yet). In this case, the generated migrations file says:
class CreateUsers < ActiveRecord::Migration def change create_table :users do |t| t.string :username t.string :password t.timestamps end end end
Let's break this down:
The first line says: Create a class called "CreateUsers" that inherits its functionality from Active Record's Migration class. That line's already a doozy, but here's the breakdown's breakdown: The Migrations class in Active Record contains helper methods that perform CRUD operations, like addtable, addcolumn, and rename_table. These are the types of things we our class (CreateUsers) to do, so it inherits those methods, giving our class the ability to perform them. Remember when naming this class to follow the Migration Naming Pattern, which Rails does for you by default.
The second line says, create a method called "change." The method name is convention in Rails for constructive migrations--aka migrations that add something to a database (in this case we're adding a whole table, but addcolumn and addindex are also constructive migrations). Since writing a Rails model will always be a constructive migration, we won't explore the other options in this article; check out Writing a Rails Migration for details on convention for destructive or change-oriented migrations.
The next lines describe the changes we want to make--we create the table called users (note the pluralization of the name we gave it, since it describes multiple users), and add columns called :username, :password. Active Record will also automatically add the timestamps columns createdat and updatedat (and it will also keep track of these for us as we add records to the table). Finally, an auto-incrementing primary key named id will be added by default, so we don't need to describe it.
With this file already created for us, we can run (back in the command line)
To create the table and update the schema.
- Second is the model file:
By default, the file Rails generates for us is:
class User < ActiveRecord::Base
attr_accessible :password, :username end Again, let's break the file down:
The file creates a User class, which as we described at the beginning of this article, is a Model class. This class is the object we'll use to perform CRUD operations on the database--it's the thing we really wanted to create to make our lives easier.
Like CreateUsers, which inherited from Active Record's Migration class, Users inherits functionality from an Active Record class: Base. Base is at the heart of Active Record: it has all the functionality for querying the database, writing to it, deleting from it, timestamping, persistence, validations--and tons of other goodies. By inheriting from Base, we've already transformed User into a rather powerful class capable of creating and destroying worlds (as long as they exist within the users table).