Adding :default => true to boolean in existing Rails column

As a variation on the accepted answer you could also use the change_column_default method in your migrations:

def up
  change_column_default :profiles, :show_attribute, true
end

def down
  change_column_default :profiles, :show_attribute, nil
end

Rails API-docs

change_column is a method of ActiveRecord::Migration, so you can't call it like that in the console.

If you want to add a default value for this column, create a new migration:

rails g migration add_default_value_to_show_attribute

Then in the migration created:

# That's the more generic way to change a column
def up
  change_column :profiles, :show_attribute, :boolean, default: true
end

def down
  change_column :profiles, :show_attribute, :boolean, default: nil
end

OR a more specific option:

def up
    change_column_default :profiles, :show_attribute, true
end

def down
    change_column_default :profiles, :show_attribute, nil
end

Then run rake db:migrate.

It won't change anything to the already created records. To do that you would have to create a rake task or just go in the rails console and update all the records (which I would not recommend in production).

When you added t.boolean :show_attribute, :default => true to the create_profiles migration, it's expected that it didn't do anything. Only migrations that have not already been ran are executed. If you started with a fresh database, then it would set the default to true.