Rails: how to require at least one field not to be blank

You can use:

validate :any_present?

def any_present?
  if %w(field1 field2 field3).all?{|attr| self[attr].blank?}
    errors.add :base, "Error message"
  end
end

EDIT: updated from original answer for Rails 3+ as per comment.

But you have to provide field names manually.
You could get all content columns of a model with Model.content_columns.map(&:name), but it will include created_at and updated_at columns too, and that is probably not what you want.