```ruby
class RemoveExtMemberships < ActiveRecord::Migration
  def self.up
    Membership.find_all_by_role(10).each{|d|d.destroy}
  end

  def self.down
  end
end
```

Refactor migration for destroying useless roles.
user
```ruby
class RemoveExtMemberships < ActiveRecord::Migration
  def self.up
    Membership.delete_all(:role => 10)
  end

  def self.down
  end
end
```
assistant