我可以用两种不同的方式运行查询来返回一个Relation.
当我询问Relation的大小时,一个查询按预期给出一个Fixnum,另一个给出一个Hash,它是Relations Group By语句中每个值的哈希值,每个值的出现次数.
在Rails 3中,我认为它总是返回一个Fixnum,因为我从来没有遇到过Rails 4的问题,它有时会返回一个Hash和一个像Rel.size.zero这样的语句？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？
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 = ?",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("-----------------------------------------------------------")
LOG
-----------------------------------------------------------
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
-----------------------------------------------------------
解决方法
ActiveRecord :: Relation对象上的大小转换为count,因为前者试图获取Relation的计数.但是当您在分组的Relation对象上调用count时,您会收到一个哈希值.
此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
这适用于following methods：计数,总和,平均值,最大值和最小值.
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?
而不是这个：
AssessmentResponse.group(:client_id).count.zero?