• Create migrations using Active Record
• Interact with a SQL database table from a Ruby class using Active Record

Active Record is magic. Well, not really. But it does build out a bunch of methods for you. For instance, when it's used properly it will give you access to methods such as create, save, and find_by. Rejoice! Never again will you have to manually build out these methods!

Active Record allows you to create a database that interacts with your class with only a few lines of code. These lines of code go to creating a model, which resides in the app/models folder, and a migration, which resides in the db/migrate folder.

The model inherits from ActiveRecord::Base while the migration inherits from ActiveRecord::Migration. Many migrations these days have a change method, but you might also see migrations with an up and a down method instead. To use Active Record, you have to stick to some specific naming conventions: while the migrations are plural, the models are singular.

When creating migrations, the class names in the migration files must match their file names. For instance, a class in the migration file called 20141013204115_create_candies.rb must be named CreateCandies while a class in a migration file called 20130915204319_add_addresses_to_houses.rb must be called AddAddressesToHouses.

You might notice that in both the examples above, the numbers at the front of the file name were ignored. These numbers are in the form YYYYMMDDHHMMSS. These timestamps are used to determine which migration should be run and in what order. For instance, if you made a table called dog_walkers and then added a column to it called rating, that would be fine as the timestamp on the dog_walkers table creation would indicate it needs to be migrated before adding the rating column to it. However, if you did this in reverse order, that is adding a column to a table that doesn't exist, then creating the table, you would get an error.

Migrations, as it was mentioned before, inherit from ActiveRecord::Migration and usually have a method called change. In change, you can create a table with the create_table method. This method automatically will create a primary key column called id, but this default can be overridden if you'd like to customize it.

Here's a simple example of the create_table method in action:

class CreateDogs < ActiveRecord::Migration[6.1]
  def change
    create_table :dogs do |t|
      t.string :name
      t.string :breed
    end
  end
end

The above code would create a table called dogs with three columns: name, breed (both explicitly created), and an implicitly created id column.

Take a look at a few data types that Active Record supports below:

Like migrations, models also inherit some code from Active Record, but they inherit from ActiveRecord::Base. A simple model would look like this:

class Dog < ActiveRecord::Base
end

Even though there are no explicit methods for retrieving name and breed, this Dog model is associated with the created dogs table above. Because of this integration, we can call name, breed, and id on any new instance of the Dog class. For example:

shiloh = Dog.new
# => #<Dog id: 1, name: nil, breed: nil>
shiloh.name = "Shiloh"
# => "Shiloh"
shiloh.breed = "Beagle"
# => "Beagle"
shiloh.save
# => true

Dog.find_by(:name => "Shiloh") == shiloh
# => true

Notice that you had access to reader and writer methods that work with the database that you never had to actually code in the model itself. You could set the name without ever writing def name=() and call the self.find_by(attribute) method without ever teaching your Dog class how to look up data in the database. It's pretty awesome. Take a look at an example below.

Let's say you wanted to make a class called Candy. Candies should have two attributes, a name (string) and the number of calories (integer). You would write the migration as seen below:

# db/migrate/20130915204319_create_candies.rb
class CreateCandies < ActiveRecord::Migration[6.1]
  def change
    create_table :candies do |t|
      t.string :name
      t.integer :calories
      t.timestamps
    end
  end
end

Note: You might be wondering what t.timestamps is doing here. Well, it creates two new columns, created_at and updated_at. These are handy columns to have around, as sometimes you want to query based on the time a record was created or updated instead of querying using attributes or ids. To read more about timestamps, go to Active Record's docs on them.

While the table name was plural, the model would be singular:

# app/models/candy.rb
class Candy < ActiveRecord::Base
end

After saving the code above, running rake db:migrate will apply the desired changes to the database by running the change method. Then you can alter the database with simple Ruby statements.

For instance, you could create three rows in the table easily:

Candy.create(name: "Milky Way Midnight", calories: 220)
Candy.create(name: "Snickers", calories: 550)
Candy.create(name: "Reese's Peanut Butter Cups", calories: 210)

Retrieving information is just as painless:

reeses = Candy.find_by(:name => "Reese's Peanut Butter Cups")
# => #<Candy id: 3, name: "Reese's Peanut Butter Cups", calories: 210>
Candy.first
# => #<Candy id: 1, name: "Milky Way Midnight", calories: 220>
snickers = Candy.find(2)
# => #<Candy id: 2, name: "Snickers", calories: 550>

As is viewing attributes:

reeses = Candy.find(3)
# => #<Candy id: 3, name: "Reese's Peanut Batter Cups", calories: 210>
reeses.calories
# => 210
reeses.name
# => "Reese's Peanut Batter Cups"

Updating information and viewing table info is also quite simple:

snickers = Candy.find(2)
# => #<Candy id: 2, name: "Snickers", calories: 550>
snickers.update(:calories => 250)
# => true

reeses = Candy.last
# => #<Candy id: 3, name: "Reese's Peanut Batter Cups", calories: 210>
reeses.update(:name => "Reeeese's Peanut Butter Cups")
# => true

Candy.find(2)
# => #<Candy id: 2, name: "Snickers", calories: 250>
Candy.last
# => #<Candy id: 3, name: "Reeeese's Peanut Butter Cups", calories: 210>

Isn't that amazing? Eleven lines of code allows you to create a table and a class that interact with each other elegantly and efficiently. It builds out methods like, create, update, count, name, calories, along with others such as build and save.

In this lab, you'll be creating the following tables: costumes, costume_stores, and haunted_houses. You'll be creating the following classes: Costume, CostumeStore, and HauntedHouse.

The costumes table will have four columns:

1. name (string)
2. price (float)
3. size (string)
4. image_url (string)

as well as the two timestamp columns: created_at and updated_at. This will provide a grand total of six columns.

The costume_stores table will have seven columns:

1. name (string)
2. location (string)
3. costume_inventory (integer)
4. num_of_employees (integer)
5. is_in_business (boolean)
6. opening_time (datetime)
7. closing_time (datetime)

The haunted_houses table will have eight columns:

1. name (string)
2. location (string)
3. theme (string)
4. price (float)
5. family_friendly (boolean)
6. opening_date (datetime)
7. closing_date (datetime)
8. description (text)

Before coding out the creation of these tables, read about Active Record below.

You will be altering code in three files in the models folder and creating three files in the db/migrate folder.

├── app
│   └── models
│       ├── costume.rb
│       ├── costume_store.rb
│       └── haunted_house.rb
└──db
    └── migrate
        ├── 001_create_costumes.rb
        ├── 002_create_costume_stores.rb
        └── 003_create_haunted_houses.rb

This is a test-driven lab so start with the first test and work your way down. Your models should be no longer than two lines of code. Most of your work will be done in the migrations.

You can use rake db:create_migration to create the files for the migrations.

• The first step is to run bundle install.
• Create the Costume class in app/models/costume.rb.
• Fill out the Active Record migration for the costumes table such that it passes the specs.
• Create the CostumeStore class in app/models/costume_store.rb.
• Fill out the Active Record migration for the costume_stores table such that it passes the specs.
• Create the HauntedHouse class in app/models/haunted_house.rb.
• Fill out the Active Record migration for the haunted_houses table such that it passes the specs.

When running your migrations and rollbacks, use RACK_ENV=test in order to run the migrations for the test environment:

$ bundle exec rake db:migrate RACK_ENV=test
$ bundle exec rake db:rollback RACK_ENV=test

You can also run the migrations without RACK_ENV=test, which will migrate the development database. This is helpful if you want to explore the code from the console:

$ bundle exec rake db:migrate
$ bundle exec rake console

Just like for any other lab, run learn test to view your test progress. However, unlike some of the other labs in this section, for this lab, when updating an existing migration, you will need to rollback your previous migrations for that table using the Rake command rake db:rollback RACK_ENV=test. Otherwise, the schema will remain unchanged and the changes you make to your migrations will not be seen.

For example, say you've run rake db:migrate RACK_ENV=test and learn test once to start, and see that you need to add an attribute to the costume_stores table. Since this table is the second migration of three, you will need to run rake db:rollback RACK_ENV=test twice to remove the previous migration for this table, then run rake db:migrate RACK_ENV=test again to update the schema. Your code, however, may break if the other migration files are empty.

• Active Record Migrations
  • Just look at the code for the example migrations
• Creating Active Record Models
• Timestamps