- 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
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.
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.
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).
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.
View Writing Our Own Migrations on Learn.co and start learning to code for free.
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