- 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.
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