复杂 ActiveRecord 案例中大小、长度和计数之间的差异答案

【问题标题】：Difference between size, length and count in complicated ActiveRecord case复杂 ActiveRecord 案例中大小、长度和计数之间的差异
【发布时间】：2012-08-10 16:04:54
【问题描述】：

[10] pry(main)> r.respondents.select(:name).uniq.size

(1.1ms)  SELECT DISTINCT COUNT("respondents"."name") FROM "respondents" 
INNER JOIN "values" ON "respondents"."id" = "values"."respondent_id" WHERE 
"values"."round_id" = 37 => 495

[11] pry(main)> r.respondents.select(:name).uniq.length

Respondent Load (1.1ms)  SELECT DISTINCT name FROM "respondents" 
INNER JOIN "values" ON "respondents"."id" = "values"."respondent_id" WHERE
"values"."round_id" = 37 => 6

为什么每个查询返回的结果不同？

【问题讨论】：

标签： sql ruby-on-rails activerecord

【解决方案1】：

.count #=> this always triggers a SELECT COUNT(*) on the database

.size #=> if the collection has been loaded, defers to Enumerable#size, else does the SELECT COUNT(*)

.length #=> always loads the collection and then defers to Enumerable#size

【讨论】：

【解决方案2】：

r.respondents.select(:name).uniq 返回一个 ActiveRecord::Relation 对象，该对象覆盖size。

见：http://api.rubyonrails.org/classes/ActiveRecord/Relation.html#method-i-size

对此类对象调用size 会检查该对象是否“已加载”。

# Returns size of the records.
def size
  loaded? ? @records.length : count
end

如果是“已加载”，则返回@records 数组的长度。否则，它会调用count，如果没有参数，它将“return a count of all the rows for the model.”

那么为什么会有这种行为呢？ AR::Relation 只有在首先调用 to_a 或 explain 时才会“加载”：

https://github.com/rails/rails/blob/master/activerecord/lib/active_record/relation.rb

load 方法上方的评论解释了原因：

# Causes the records to be loaded from the database if they have not
# been loaded already. You can use this if for some reason you need
# to explicitly load some records before actually using them. The
# return value is the relation itself, not the records.
#
#   Post.where(published: true).load # => #<ActiveRecord::Relation>
def load
  unless loaded?
    # We monitor here the entire execution rather than individual SELECTs
    # because from the point of view of the user fetching the records of a
    # relation is a single unit of work. You want to know if this call takes
    # too long, not if the individual queries take too long.
    #
    # It could be the case that none of the queries involved surpass the
    # threshold, and at the same time the sum of them all does. The user
    # should get a query plan logged in that case.
    logging_query_plan { exec_queries }
  end

  self
end

因此，也许使用AR::Relation#size 来衡量此关系上查询的潜在复杂性的大小，其中length 回退到返回记录的计数。

【讨论】：

【解决方案3】：

在将 Rails 3.2 转换为 4.1 时，似乎 AR::Relation#size 有所不同。以前它返回“行”的数量，而（在我的情况下）它现在返回一个哈希。更改为使用 #count 似乎与 3.2 中的 #size 给出相同的结果。我在这里有点含糊，因为在 4.1 上的“rails 控制台”中运行测试在 4.1 上通过“rails 服务器”运行时并没有给出相同的结果

【讨论】：