Mongoid : migrate an existing String field to a localized field

20160929132327_localize_species.rb

# db/migrate/20160929132327_localize_species.rb

class LocalizeSpecies < Mongoid::Migration
  def self.up
    migrate_field_to_localized(Species, :name)
  end

  def self.down
  end
end

mongoid_migration.rb

# config/initializers/mongoid_migration.rb

module Mongoid
  class Migration
    def self.migrate_field_to_localized(klass, field_name)
      klass.all.each do |record|
        next unless record[field_name].is_a?(String)

        old_value = record[field_name]
        record.unset(field_name)
        record.send("#{field_name}=", old_value)

        record.save
      end
    end
  end
end

When going from a classical String field in Mongoid to a localized field (with localize: true option), it can be painful to migrate the existing records.

This is a small add-on to mongoid_rails_migrations I use to easily migrate the records.