Rails 模型：如何在保存之前计算（使用过滤器）嵌套实体

Question

I have a parent entity that contains children entities which are all created at the same time through nested attributes. On the parent entity, I have a callback before save that computes some data to store, based on the nested items. The problem is that count triggers a request to the database but entities don't exist yet so it returns 0. If it was just about counting, I'd use the size property, but I need to filter what I count on stuff itself computed in a callback... What's the best solution? See the example below.

PS: I know about counter_cache for the count but since I only create children through the parent, I prefer to compute it only once at the same time as the other property I compute.

class Parent < ApplicationRecord
  has_many :children
  accepts_nested_attributes_for :children

  before_save :update_total

  private

  def update_total
    self.total = children.size # OK, this works
    self.total_success = children.count(&:result) # Always 0 since it queries the DB
  end
end

class Child < ApplicationRecord
  belongs_to :parent

  before_save :update_result

  private

  def update_result
    self.result = check_result
  end
end

Answer 1

For now, I've found something that works: the children before save callback is called before the parent's one. I don't feel it's great but at least understandable.

I update the total_result this way:

self.total_success = 0
children.each { |child| self.total_success += 1 if child.result }