Question

[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区别？

Answer 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

Answer 2

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

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

在这样的对象上调用size检查对象是否“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，其中没有参数，将为“return a count of all the rows for the model.”

那为什么会这样呢？如果首先调用to_a或explain，则只会“加载”AR :: Relation：

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

为什么会在load方法上方的评论中解释：

# 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，使用AR::Relation#size可能是衡量此关系查询潜在复杂程度的大小，其中length会回退到返回记录的计数。

Answer 3

将Rails 3.2转换为4.1时，AR :: Relation＃size似乎不同。以前它返回了＆＃34;行的数量＆＃34;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具有相同的结果。我在这里做了一点模糊，因为他们在＆＃39; rails console＆＃39;在通过＆＃;; rails server＆＃39;运行时，4.1上没有给出相同的结果在4.1

复杂的ActiveRecord案例中大小，长度和计数之间的差异

3 个答案: