Migrations and Active Record Lab

Contents

Section
Objectives
Active Record
Example
Instructions
Resources

Objectives

🎃👻🎃

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

2. price

3. size

4. image url

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

2. location

3. number of costumes, or "costume inventory"

4. number of employees

5. whether or not it's still in business

6. opening time

7. closing time

The haunted_houses table will have eight columns:

1. name

2. location

3. theme

4. price

5. whether they're family friendly or not

6. opening date

7. closing date

8. long description

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

Active Record Review

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.

Migrations

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. Later on, these timestamps will become important as Rails uses them 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.

NOTE: As of Active Record 5.x, we can no longer inherit directly from ActiveRecord::Migration and must instead specify which version of Active Record / Rails the migration was written for. If we were writing a migration for Active Record 5.1, we would inherit from ActiveRecord::Migration[5.1]. Don't worry too much about this until you get to the Rails section. Until then, if you encounter an error like this...

StandardError: Directly inheriting from ActiveRecord::Migration is not supported. Please specify the Rails release the migration was written for:

  class CreateDogs < ActiveRecord::Migration[4.2]

...simply add [4.2] to the end of ActiveRecord::Migration, exactly as the error message instructs.

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

class CreateDogs < ActiveRecord::Migration[4.2]
  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:

Data Type	Examples
boolean	true, false
integer	2, -13, 485
string	"Halloween", "Boo!", strings between 1-255 characters
datetime	DateTime.now, DateTime.new(2014,10,31)
float	2.234, 32.2124, -6.342
text	strings between 1 and 2 ^ 32 - 1 characters

Models

Like migrations, models also inherit, 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 cooperated with the database that you never had to actually code. 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.

Example

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/migrations/20130915204319_create_candies.rb

class CreateCandies < ActiveRecord::Migration[4.2]
  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 of creation or update-tion instead of querying using attributes or ids. To read more about timestamps, go to Active Record's docs on them.

While the migration 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.

Instructions

File Structure

You will only be altering code in six files, the three files in the models folder and the three files in the db/migrations folder.

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

Getting Started

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.

• The first step is to run bundle install.

• Create the Costume class in app/models/.

• Fill out the Active Record migration for costumes such that it passes the specs.

• Create the CostumeStore class in app/models/.

• Fill out the Active Record migration for costume_stores such that it passes the specs.

• Create the HauntedHouse class in app/models/.

• Fill out the Active Record migration for haunted_houses such that it passes the specs.

Just like for any other lab, run learn to view your test progress. However, unlike some of the other labs in thie 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. 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 and learn 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 twice to remove the previous migration for this table, then run rake db:migrate again to update the schema.

Resources

• Active Record Migrations

  • Just look at the code for the example migrations

• Creating Active Record Models
• Timestamps

View Active Record Costume Store on Learn.co and start learning to code for free.