为什么Relation.size有时会在Rails 4中返回哈希

Question

我可以以两种不同的方式运行一个查询来返回一个关系。

当我询问关系的大小时，一个查询按预期给出一个Fixnum，另一个查询给出一个Hash，它是Relations语句中每个值的散列，其中包含每个值的出现次数。

在Rails 3中，我假设它总是返回Fixnum，因为我从来没有遇到过问题，Rails 4有时会返回哈希和类似于Rel.size.zero的语句？给出错误：

未定义的方法“零？”用于{}：散列

我最好只使用.blank吗？方法以检查零记录以确保避免意外错误？

下面是用于这两个查询和结果日志的带有looging语句的代码片段

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在ActiveRecord::Relation对象上转换为count，因为前者试图获取Relation的计数。但是，当您对一个分组的count对象调用Relation时，您将收到一个哈希。

此散列的键是分组列的值；此散列的值是相应的计数。

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是这样的：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?

而不是这样：

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