3 
[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内容不同？

4

3 回答 3

6 
.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
于 2012-08-10T19:16:30.130 回答
1

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

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

调用size此类对象会检查该对象是否“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，它调用count，不带参数，将“返回模型所有行的计数” 。

那么为什么会有这种行为呢？to_a一个 AR::Relation 只有在先调用或时才被“加载” explain：

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

方法上方的评论中解释了原因load：

# 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 usingAR::Relation#size是对该关系的查询的潜在复杂性大小的度量，其中length回退到返回记录的计数。

于 2012-08-10T18:34:59.317 回答
0

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（在我的例子中）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

于 2014-12-25T20:30:56.137 回答