Ancestry - how to get all parent without children?

问题

I am doing it this way:

@disabled_options = []
Category.where('ancestry is NULL').each do |cat|
  @disabled_options << cat.id if cat.has_children?
end

Is there any more elegant way to get all parent without children?

回答1:

Category.where("id IN (SELECT parent_id FROM categories)")

Assuming parent_id is a field pointing to the parent category.

This will select those categories that are pointed to using "parent_id", so if there's a child, the child will have "parent_id" set, therefore the category referenced to by "parent_id" has children.

回答2:

This one-liner may help you.

Category.where(id: Category.pluck(:ancestry).compact.map { |e| e.split('/') }.flatten.uniq)

来源：https://stackoverflow.com/questions/11366663/ancestry-how-to-get-all-parent-without-children