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

Question

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) FIRST: Add column with default value

2) THEN: 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