Ruby on Rails: How do I add a not null constraint to an existing column using a migration?

In my Rails (3.2) app, I have a bunch of tables in my database but I forgot to add a few not null constraints. How can I write a migration which adds not null to an existing column?

You can also use change_column_null:

change_column_null :table_name, :column_name, false

For Rails 4+, nates' answer (using change_column_null) is better.

Pre-Rails 4, try change_column.

1. Add column with default value

2. Remove default value

add_column :orders, :items, :integer, null: false, default: 0
change_column :orders, :items, :integer, default: nil

If you are using it on a new create migration script/schema here is how we can define it

class CreateUsers < ActiveRecord::Migration[5.2]
  def change
    create_table :users do |t|
    t.string :name, null: false     # Notice here, NOT NULL definition
    t.string :email, null: false
    t.string :password, null: false
    t.integer :created_by
    t.integer :updated_by 

    t.datetime :created_at
    t.datetime :updated_at, default: -> { 'CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP' }
   end
  end
end

In my approach, I add NOT NULL constraint to columns i need in my existing migrated migration. After that, I reset all my migrations by using this command:

> rake db:migrate:reset

This will drop the database, create it again and run all the migrations. You can check your changes in schema.rb.

If you have few columns in simple migrations, you can use this approach.