To add multiple columns to a table, separate field:type pairs with spaces when using rails generate migration command.
The general syntax is:
rails generate migration NAME [field[:type][:index] field[:type][:index]] [options]
For example, the following will add name, salary and email fields to the users table:
rails generate migration AddDetailsToUsers name:string salary:decimal email:string
Which generates the following migration:
class AddDetailsToUsers < ActiveRecord::Migration[5.0]
def change
add_column :users, :name, :string
add_column :users, :salary, :decimal
add_column :users, :email, :string
end
end