Question

我有一个方法可以计算模型中许多浮点属性的统计数据（主要是总和）。

模特

class GroupPlayer < ActiveRecord::Base
  belongs_to :group
  has_many :scored_rounds
  has_many :rounds, dependent: :destroy
end

class Round < ActiveRecord::Base
  belongs_to :group_player
end

class ScoredRound < Round
  # STI
end

提供从其他方法调用的最多4个浮点属性的统计信息的方法，具体取决于我是否获得一个玩家或一组玩家的统计数据。 ScoredRound上的初始过滤器传递给方法（sr）

def method_stats(method,sr,grp)
  rounds = sr.where.not(method => nil)
  number_rounds = rounds.count
  won = rounds.sum(method).round(2)
  if method == :quality
    dues = grp.options[:dues] * number_rounds
  else
    dues = grp.options["#{method.to_s}_dues"] * number_rounds
  end
  balance = (won - dues).round(2)
  perc = dues > 0 ? (won / dues).round(3) : 0.0
  [self.full_name,number_rounds,won,dues,balance,perc]
end

我在ScoredRounds中总结的4个属性中的3个可能没有设置（nil）如果玩家没有赢得该游戏，那么轮次被过滤。

一切正常，直到我决定增加使用轮数的限制。例如，如果我只希望传递给method_stats的查询中最后25轮的状态，请致电：

def money_stats(grp,method,limit=100)
  sr = self.scored_rounds.where.not(method => nil).order(:date).reverse_order.limit(limit)
  method_stats(method,sr,grp)
end

同样，我刚刚在查询中添加了limit和order子句。所有记录都很好。

如果我在控制台中模拟程序而没有使用上述方法（或使用它们！）我会得到一个错误的总和

gp = GroupPlayer.find(123)
  GroupPlayer Load (2.1ms)  SELECT  "group_players".* FROM "group_players" WHERE "group_players"."id" = $1 LIMIT $2  [["id", 123], ["LIMIT", 1]]
  => valid group player

sr = gp.scored_rounds.where.not(:quality => nil)
  ScoredRound Load (1.7ms)  SELECT "rounds".* FROM "rounds" WHERE "rounds"."type" IN ('ScoredRound') AND "rounds"."group_player_id" = $1 AND ("rounds"."quality" IS NOT NULL)  [["group_player_id", 123]]

  => #<ActiveRecord::AssociationRelation [#<ScoredRound id: 5706, player_id: 123, group_player_id: 123, event_id: 12, type: "ScoredRound", date: "2016-11-04", team: 3, tee: "White", quota: 32, front: 15, back: 15, total: 30, created_at: "2016-11-04 14:18:27", updated_at: "2016-11-04 19:12:47", quality: 0.0, skins: nil, par3: nil, other: nil>,...]

sr.count
   (1.5ms)  SELECT COUNT(*) FROM "rounds" WHERE "rounds"."type" IN ('ScoredRound') AND "rounds"."group_player_id" = $1 AND ("rounds"."quality" IS NOT NULL)  [["group_player_id", 123]]
  => 44

sr.sum(:quality)
   (1.0ms)  SELECT SUM("rounds"."quality") FROM "rounds" WHERE "rounds"."type" IN ('ScoredRound') AND "rounds"."group_player_id" = $1 AND ("rounds"."quality" IS NOT NULL)  [["group_player_id", 123]]
  => 354.166666666667

# Now if I add the order and limit clause

sr = gp.scored_rounds.where.not(:quality => nil).order(:date).reverse_order.limit(25)
  ScoredRound Load (1.6ms)  SELECT  "rounds".* FROM "rounds" WHERE "rounds"."type" IN ('ScoredRound') AND "rounds"."group_player_id" = $1 AND ("rounds"."quality" IS NOT NULL) ORDER BY "rounds"."date" DESC LIMIT $2  [["group_player_id", 123], ["LIMIT", 25]]
  => => #<ActiveRecord::AssociationRelation [...]

sr.count
   (1.1ms)  SELECT COUNT(count_column) FROM (SELECT  1 AS count_column FROM "rounds" WHERE "rounds"."type" IN ('ScoredRound') AND "rounds"."group_player_id" = $1 AND ("rounds"."quality" IS NOT NULL) LIMIT $2) subquery_for_count  [["group_player_id", 123], ["LIMIT", 25]]
=> 25

sr.sum(:quality)
   (1.8ms)  SELECT  SUM("rounds"."quality") FROM "rounds" WHERE "rounds"."type" IN ('ScoredRound') AND "rounds"."group_player_id" = $1 AND ("rounds"."quality" IS NOT NULL) LIMIT $2  [["group_player_id", 123], ["LIMIT", 25]]
=> 354.166666666667

###  This is the error, it return the sum off all records, 
# not the limited???? if I use pluck and sum

sr.pluck(:quality)
=> [10.0, 11.3333333333333, 10.0, 34.0, 0.0, 7.33333333333333, 0.0, 0.0, 31.5, 0.0, 21.3333333333333, 0.0, 19.0, 0.0, 0.0, 7.5, 0.0, 20.0, 10.0, 28.0, 8.0, 9.5, 0.0, 3.0, 24.0]

sr.pluck(:quality).sum
=> 254.49999999999994

不知道我是否在AREL中发现了一个错误，或者我做错了什么。我用Round而不是STI ScoredRound尝试了相同的结果。

有什么想法吗？

Answer 1

如果您注意到，SUM的{{1}}结果与LIMIT一样，都是相同的：

sr = gp.scored_rounds.where.not(:quality => nil)
sr.sum(:quality)
   (1.0ms)  SELECT SUM("rounds"."quality") FROM "rounds" WHERE "rounds"."type" IN ('ScoredRound') AND "rounds"."group_player_id" = $1 AND ("rounds"."quality" IS NOT NULL)  [["group_player_id", 123]]
  => 354.166666666667

sr = gp.scored_rounds.where.not(:quality => nil).order(:date).reverse_order.limit(25)
sr.sum(:quality)
   (1.8ms)  SELECT  SUM("rounds"."quality") FROM "rounds" WHERE "rounds"."type" IN ('ScoredRound') AND "rounds"."group_player_id" = $1 AND ("rounds"."quality" IS NOT NULL) LIMIT $2  [["group_player_id", 123], ["LIMIT", 25]]
=> 354.166666666667

因为LIMIT会影响查询返回的行数而SUM只返回一行，所以该函数适用于所有44条记录，而不是{ {1}}。这不是LIMIT发生的事情，它只适用于查询返回的25条记录。

不知道我是否在AREL中发现了错误或我做错了什么

可悲的是，99.9％的时间不是错误，而是我们的错:(

Answer 2

# File activerecord/lib/active_record/relation/calculations.rb, line 75
def sum(column_name = nil)
  return super() if block_given?
  calculate(:sum, column_name)
end

如果您致电sr.sum(:quality)，则将质量汇总为列名，并计算给定列的值总和。

如果关联具有限制条款

2 个答案: