Rails migrations - add default value to existing column
Here's how to update an existing column to use a default value
If we don’t supply a default value to a column Rails will default it to be
nil. A lot of the time this is fine, say a user hasn’t supplied a optional attribute then it doesn’t really matter whether it’s set or not but say we have an Article model and we want to set a enumerable
status against it, say
published, it would be good to have a default. Generally in this case if an admin were creating a new article we would want the
status to be
draft so it isn’t visible in a half finished state. When we’re first creating our system sometimes these requirements only present themselves over time and we have to go back to update them. So let’s update our articles to default to a
How to update the default value from a column using a Rails migration?
First of all run a migration like
rails generate migration UpdateStatusDefaultToDraftArticles. Then update the migration to have this:
class UpdateStatusDefaultToDraftArticles < ActiveRecord::Migration[6.1] def change change_column_default :articles, :status, from: nil, to: 'draft' end end
This updates the columns default value from
'draft' meaning any new articles creating will have a default
to: is best as it’s more explanatory and allows the migration to be reversed if your requirements change but you could write
change_column_default :articles, :status, 'draft' instead.
How to drop the default value from a column using a Rails migration?
If you wanted to drop the default value you can run:
class UpdateDefaultOnStatusArticles < ActiveRecord::Migration[6.1] def change change_column_default :articles, :status, nil end end
This will drop whatever default was against
status and set it to