大小，长度和复杂ActiveRecord的情况下计算的区别 [英] Difference between size, length and count in complicated ActiveRecord case

问题描述

  [10]撬（主）＆GT; r.respondents.select（：名称）.uniq.size

（1.1ms）SELECT DISTINCT COUNT（答卷。域名）从答卷
INNER JOIN值ON答卷。ID=值。respondent_idWHERE
值，round_id= 37 =＆GT; 495

[11]撬（主）＆GT; r.respondents.select（：名称）.uniq.length

投诉负载（1.1ms）SELECT DISTINCT的名字从答卷
INNER JOIN值ON答卷。ID=值。respondent_idWHERE
值，round_id= 37 =＆GT; 6
 

为什么在了每个查询返回的区别是什么？

解决方案

  .Count之间＃=＆GT;这个总是在数据库上触发一个SELECT COUNT（*）

.size＃=＆GT;如果集合已经被加载，推迟到可枚举＃大小，否则不会在SELECT COUNT（*）

.length＃=＆GT;总是加载的集合，然后委托给可枚举＃大小
 

[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

Why the difference in what each query returns?

解决方案

.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

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