Rails关联对象的最大计数？

Question

我需要一个查询的帮助。我有多个食堂，每个食堂都有多餐饭，每顿饭都有多个MealPicks。

虽然我不知道这个MealPick模型是否是个好主意，因为我需要显示今天挑选了多少次饭，所以我需要时间戳来进行这个查询。

class Meal < ActiveRecord::Base
  def todays_picks
    meal_picks.where(["created_at >= ? AND created_at < ?", Date.today.beginning_of_day, Date.today.end_of_day])
  end
end

之前，我只有一个meal_picked_count计数器在餐中，我增加了increment_counter方法。

好的，现在我需要为每个食堂展示一顿MealPicks最多的饭菜，我在控制台里玩了一遍，尝试了一些类似Canteen.find(1).meals.maximum("meal_picks.count")的东西，但这显然是行不通的，因为它不是一个专栏。

有什么想法吗？

Answer 1

canteen.meals.max {|m| m.meal_picked_count}

Answer 2

你可以这样做：

MealPick.joins(:meal => :canteen)
        .where("canteens.id = ?", 1)
        .order("count_all DESC")
        .group(:meal_id)
        .count

它将返回这样的有序散列：

{ 200 => 25 }

在那里，200将是餐id，25将是计数。

更新

对于感兴趣的人，我开始玩这个游戏，看看是否可以使用ActiveRecord的子查询来提供比我之前想出的更有意义的信息。我现在拥有的是：

class Meal < ActiveRecord::Base
  belongs_to :canteen
  has_many :meal_picks
  attr_accessible :name, :price
  scope :with_grouped_picks, ->() {
    query = <<-QUERY
      INNER JOIN (#{Arel.sql(MealPick.counted_by_meal.to_sql)}) as top_picks 
      ON meals.id = top_picks.meal_id
    QUERY

    joins(query)
  }

  scope :top_picks, with_grouped_picks.order("top_picks.number_of_picks DESC")
  scope :top_pick, top_picks.limit(1)
end

class MealPick < ActiveRecord::Base
  belongs_to :meal
  attr_accessible :user

  scope :counted_by_meal, group(:meal_id).select("meal_id, count(*) as number_of_picks")
  scope :top_picks, counted_by_meal.order("number_of_picks DESC")
  scope :top_pick, counted_by_meal.order("number_of_picks DESC").limit(1)

end

class Canteen < ActiveRecord::Base
  attr_accessible :name
  has_many :meals
  has_many :meal_picks, through: :meals

  def top_picks
    @top_picks ||= meals.top_picks
  end

  def top_pick
    @top_pick ||= top_picks.first
  end
end

这使我可以这样做：

c = Canteen.first
c.top_picks #Returns their meals ordered by the number of picks
c.top_pick  #Returns the one with the top number of picks

假设我想按挑选的次数来订购所有的饭菜。我可以这么做：

Meal.includes(:canteen).top_picks #Returns all meals for all canteens ordered by number of picks.
Meal.includes(:canteen).where("canteens.id = ?", some_id).top_picks #Top picks for a particular canteen
Meal.includes(:canteen).where("canteens.location = ?", some_location) #Return top picks for a canteens in a given location

由于我们使用的是联接、分组和服务器端计数，所以不需要加载整个集合来确定挑选计数。这有点灵活，而且可能更有效率。