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

Question

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.

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

I'm not sure when this was written, but currently to add or remove a default from a column in a migration, you can use the following:

change_column_null :products, :name, false

Rails 5:

change_column_default :products, :approved, from: true, to: false

http://edgeguides.rubyonrails.org/active_record_migrations.html#changing-columns

Rails 4.2:

change_column_default :products, :approved, false

http://guides.rubyonrails.org/v4.2/active_record_migrations.html#changing-columns

Which is a neat way of avoiding looking through your migrations or schema for the column specifications.

If you just made a migration, you can rollback and then make your migration again.

To rollback you can do as many steps as you want:

rake db:rollback STEP=1

Or, if you are using Rails 5.2 or newer:

rails db:rollback STEP=1

Then, you can just make the migration again:

def change
  add_column :profiles, :show_attribute, :boolean, default: true
end

Don't forget to rake db:migrate and if you are using heroku heroku run rake db:migrate

Also, as per the doc:

default cannot be specified via command line

https://guides.rubyonrails.org/active_record_migrations.html

So there is no ready-made rails generator. As specified by above answers, you have to fill manually your migration file with the change_column_default method.

You could create your own generator: https://guides.rubyonrails.org/generators.html