rails how to add new column to existing code example

Example 1: how to add column to table rails

rails g migration add_description_to_users description:string
#'g' means generate, the 'add' in 'add_description_to_users' tells
#Rails you want to add a column, the 'description' is the new column name,
#and the 'users' is the table you want to add it to, must be plural. 
#After you've checked your migration to make sure its what you want...
rails db:migrate

Example 2: rails add column next to

add_column :patient_allergies, :reaction_id, :integer, after: :patient_id