Let's Write Some Migrations

Objectives

Write your own migrations
Run a migration to create a table
Run a migration to add a column to a table
Run a migration to change something in the table

Creating a Table

The first thing we will do is create a table. In db/migrate/01_create_students.rb, write the code to create a table with Active Record. We've created a class for you called CreateStudents.

Define a method called change and use the Active Record create_table method within that method to create the table. The table should have a :name column with a type string.

After you finish defining the change method, run the migrations by running rake db:migrate in your terminal.

Adding a Column

The next thing we will do is add a couple of columns to the students table we just created. To do this, we will create a second migration file. We cannot add these columns to the existing file. Let's call our new file 02_add_grade_and_birthdate_to_students.rb. It should live in db/migrate just like the first migration.

This new migration will look similar to the previous one. We will need a class that inherits from ActiveRecord::Migration, and we will need to define a change method. Sticking to conventions, name the class AddGradeAndBirthdateToStudents, since that is what we're doing (and that is the camel case version of the filename, minus the numbers in front). Inside #change, instead of create_table, we will use the add_column Active Record method.

Let's add a :grade column and a :birthdate column. The :grade column type should be integer and the :birthdate column type should be string.

Changing a Column

Imagine you're creating an incredible web app to send out a birthday greeting on each student's birthday. While building this, you realize you accidentally stored your birthdate data as a string. It would be much easier to work with if the column type was datetime instead. Let's fix that.

Finally, we will change a column type, string to datetime. Same as before, you'll have to create another migration file. This time call it 03_change_datatype_for_birthdate.rb. Once again, name the class the same name as the file but with capital letters instead of underscores: ChangeDatatypeForBirthdate.

This migration will have the same setup as the last. Be sure to use the change_column method. It takes three necessary arguments: change_column(table_name, column_name, type).

Active Record 5.x Migration Syntax

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[5.2]

...simply add [5.2] to the end of ActiveRecord::Migration, exactly as the error message instructs.