Specifying default value from lambda in a migration

Question

I want to add a non-null column unique_id to some of my models. The implementation for this would basically be SecureRandom.hex(8). Is there a way to specify this in the migration itself, such that it computes the default values for all the older rows.

Maybe something like this:

add_column :users, :unique_id, :string, null: false, default: proc { generate_unique_id }.call

The problem is, the above method will generate unique_id only once (which is kind of expected). Is there a way to specify for generating default for every row?

Answer 1

This is not possible directly in add_column. The common approach would be:

• create a nullable column;
• fill it with generated uids;
• update the column definition.

In your code, proc { generate_unique_id }.call makes not much sense, since it is an absolute equavalent of generate_unique_id.

 add_column :users, :unique_id, :string, null: true
 # it’s still a plain ruby code!
 User.find_each do |u|
   # or better use built-in SQL functoin to do this in batch
   #    inside your db, instead of performing a dozillion of updates
   u.update_column(:unique_id, generate_unique_id)
 end
 change_column :users, :unique_id, :string, null: false # NOW!