ActiveRecord, how to select empty fields

Question

I'm trying to count all User with empty last_name.

I'm trying those but they don't work

User.where("last_name = ?", "").count
User.where("last_name = ?", nil).count

Calling all against nil works but I still need to select those who's last_name are empty.

User.where("last_name != ?", nil).count

Answer 1

User.where(:last_name => nil).count

Or using ARel:

User.where(User.arel_table[:last_name] == nil).count

Answer 2

For mysql maybe:

User.where("last_name is null").count

otherwise maybe:

User.find_all_by_last_name(nil).count