```ruby
class CreateCustomers < ActiveRecord::Migration
  def change
    create_table :customers do |t|
      t.string :first_name, null: false
      t.string :last_name
      t.string :email, null: false
      t.string :phone_number
      t.string :city
      t.string :country

      t.timestamps null: false
    end
  end
end
```

Edit migration to remove not_null constraint from customer
user
```ruby
class CreateCustomers < ActiveRecord::Migration
  def change
    create_table :customers do |t|
      t.string :first_name
      t.string :last_name
      t.string :email, null: false
      t.string :phone_number
      t.string :city
      t.string :country

      t.timestamps null: false
    end
  end
end
```
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