ruby-on-rails-4 - 为什么Relation.size有时会在Rails 4中返回哈希

Question

我可以用两种不同的方式运行查询以返回一个Relation。

当我查询Relation的大小时，一个查询给出了一个Fixnum，另一个查询给出了一个Hash，它是Relations Group By语句中每个值的哈希以及每个值的出现次数。

在Rails 3中，我假设它总是返回一个Fixnum，因为我从没有遇到过Rails 4的问题，有时它会返回Hash和类似Rel.size.zero的语句？给出错误:

undefined method `zero?' for {}:Hash

我最好只使用.blank吗？检查零记录以确保避免意外错误的方法？

这是一段代码，其中包含针对两个查询的掠夺性语句以及生成的日志

代码:

assessment_responses1=AssessmentResponse.select("process").where("client_id=? and final = ?",self.id,false).group("process")
logger.info("-----------------------------------------------------------")
logger.info("assessment_responses1.class = #{assessment_responses1.class}")
logger.info("assessment_responses1.size.class = #{assessment_responses1.size.class}")
logger.info("assessment_responses1.size value = #{assessment_responses1.size}")

logger.info("............................................................")

assessment_responses2=AssessmentResponse.select("distinct process").where("client_id=? and final = ?",self.id,false)
logger.info("assessment_responses2.class = #{assessment_responses2.class}")
logger.info("assessment_responses2.size.class = #{assessment_responses2.size.class}")
logger.info("assessment_responses2.size values = #{assessment_responses2.size}")
logger.info("-----------------------------------------------------------")

日志

-----------------------------------------------------------
assessment_responses1.class = ActiveRecord::Relation::ActiveRecord_Relation_AssessmentResponse
   (0.5ms)  SELECT COUNT(`assessment_responses`.`process`) AS count_process, process AS process FROM `assessment_responses` WHERE `assessment_responses`.`organisation_id` = 17 AND (client_id=43932 and final = 0) GROUP BY process
assessment_responses1.size.class = Hash
  CACHE (0.0ms)  SELECT COUNT(`assessment_responses`.`process`) AS count_process, process AS process FROM `assessment_responses` WHERE `assessment_responses`.`organisation_id` = 17 AND (client_id=43932 and final = 0) GROUP BY process
assessment_responses1.size value = {"6 Month Review(1)"=>3, "Assessment(1)"=>28, "Assessment(2)"=>28}
............................................................
assessment_responses2.class = ActiveRecord::Relation::ActiveRecord_Relation_AssessmentResponse
   (0.5ms)  SELECT COUNT(distinct process) FROM `assessment_responses` WHERE `assessment_responses`.`organisation_id` = 17 AND (client_id=43932 and final = 0)
assessment_responses2.size.class = Fixnum
  CACHE (0.0ms)  SELECT COUNT(distinct process) FROM `assessment_responses` WHERE `assessment_responses`.`organisation_id` = 17 AND (client_id=43932 and final = 0)
assessment_responses2.size values = 3
-----------------------------------------------------------

Answer 1

size对象上的ActiveRecord::Relation转换为count，因为前者试图获取Relation的计数。但是，当您在分组的count对象上调用Relation时，会收到一个哈希。

该哈希的键是分组列的值；此哈希的值是各自的计数。

AssessmentResponse.group(:client_id).count # this will return a Hash
AssessmentResponse.group(:client_id).size # this will also return a Hash

对于 following methods来说确实如此: count， sum， average， maximum和 minimum。

如果要检查是否存在行，只需使用 exists?，即执行以下操作:

AssessmentResponse.group(:client_id).exists?

代替这个:

AssessmentResponse.group(:client_id).count.zero?