Writing Migrations

Learning Goals

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

Instructions

In this lesson, you'll get practice writing migration code and creating new migrations. Note that in order for the tests to work, the migrations must be created with the file names as described below, using numbers instead of timestamps.

Make sure to also follow proper naming conventions and use snake_case for the file names, and PascalCase for the class names. The names must match exactly in order for the migrations to work.

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. Check the Active Record Migration docs for more help with the syntax.

After you finish defining the change method, run the migrations by running:

$ bundle exec rake db:migrate

Check the status of your migration and verify the schema was updated correctly to include a students table with a name column before proceeding.

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.

Note: While we generally recommend using rake db:create_migration to create the migration files, for this lab you'll need to create the file name manually to ensure that the tests are able to find a file with the correct name.

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. It takes three necessary arguments: add_column(table_name, column_name, type). Check the Active Record Migration docs for more help with the syntax.

Let's add two columns. You'll need to call #add_column twice: once for each column you're adding. Add a grade column that is an integer, and a birthdate column that is a string.

After you finish defining the change method, run the migrations by running:

$ bundle exec rake db:migrate

Check the status of your migration and verify the schema was updated correctly before proceeding. The students table should now have name, grade, and birthdate columns.

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. Write a #change method, as usual. This time, in the #change method, be sure to use the #change_column method. It takes three necessary arguments: change_column(table_name, column_name, type). Check the Active Record Migration docs for more help with the syntax.

After you finish defining the #change method, run the migrations by running:

$ bundle exec rake db:migrate

Check the status of your migration and verify the schema was updated correctly before proceeding. The data type for the birthdate should now be datetime.