1. Understand the connection between an ORM and Active Record
2. Understand why Active Record is useful
3. Develop a basic understanding of how to get started with Active Record

By now you are familiar with the concept of an ORM, an Object-Relation Mapper, and should have written something of your own in the Student and InteractiveRecord classes. Our latest iteration was our most powerful yet, it could give us lots of functionality via inheritance.

While building your own ORM for a single Class is a great way to learn about how object-oriented programming languages commonly interact with a database, imagine you had many more classes. To test and maintain custom code for each project we work on would distract our attention from making cool stuff to building database connectivity. To save themselves and other developers this headache, the ActiveRecord Ruby gem team built the ActiveRecord gem.

In this lesson we'll read about how to to have ActiveRecord link our Ruby models with rows in a database table. We won't write the code yet, but we'll familiarize ourself with common code blocs used in ActiveRecord-using projects.

Active Record is a Ruby gem, meaning we get an entire library of code just by running gem install activerecord or by including it in our Gemfile.

Once our Gem environment knows to put ActiveRecord into the picture, we need to tell ActiveRecord where the database is located that it will be working with.

We do this by running ActiveRecord::Base.establish_connection. Once establish_connection is run, ActiveRecord::Base keeps it stored as a class variable at ActiveRecord::Base.connection.

NOTE: If you'd like to type along in an IDE environment, you can experiment by using IRB with: irb -r active_record provided you've installed ActiveRecord with gem install activerecord

ActiveRecord::Base.establish_connection(
  :adapter => "sqlite3",
  :database => "db/students.sqlite"
)

But our database is empty. Let's create a table to hold students.

Let's create our table using SQL:

sql = <<-SQL
  CREATE TABLE IF NOT EXISTS students (
  id INTEGER PRIMARY KEY,
  name TEXT
  )
SQL

# Remember, the previous step has to run first so that `connection` is set!
ActiveRecord::Base.connection.execute(sql)

The last step is to tell your Ruby class to make use of ActiveRecord's built-in ORM methods. With Active Record and other ORMs, this is managed through Class Inheritance. We simply make our class (Student) a subclass of ActiveRecord::Base.

class Student < ActiveRecord::Base
end

Our Student class is now our gateway for talking to the students table in the database. The Student class has gained a whole bunch of new methods via its inheritance relationship to ActiveRecord. Let's look at a few of them

Retrieve a list of all the columns in the table:

Student.column_names
#=> [:id, :name]

Create a new Student entry in the database:

Student.create(name: 'Jon')
# INSERT INTO students (name) VALUES ('Jon')

Retrieve a Student from the database by id:

Student.find(1)

Find by any attribute, such as name:

Student.find_by(name: 'Jon')
# SELECT * FROM students WHERE (name = 'Jon') LIMIT 1

You can get or set attributes of an instance of Student once you've retrieved it:

student = Student.find_by(name: 'Jon')
student.name
#=> 'Jon'

student.name = 'Steve'

student.name
#=> 'Steve'

And then save those changes to the database:

student = Student.find_by(name: 'Jon')
student.name = 'Steve'
student.save

Note that our Student class doesn't have any methods defined for #name either. Nor does it make use of Ruby's built-in attr_accessor method.

class Student < ActiveRecord::Base
end

You've now seen how ActiveRecord creates a link between Ruby and databases.

View Active Record Mechanics (CRUD) on Learn.co and start learning to code for free.