Why does Relation.size sometimes return a Hash in Rails 4

Question

I can run a query in two different ways to return a Relation.

When I interrogate the size of the Relation one query gives a Fixnum as expected the other gives a Hash which is a hash of each value in the Relations Group By statement with the number of occurrences of each.

In Rails 3 I assume it always returned a Fixnum as I never had a problem whereeas with Rails 4 it sometimes returns a Hash and a statement like Rel.size.zero? gives the error:

undefined method `zero?' for {}:Hash

Am I best just using the .blank? method to check for zero records to be sure of avoiding unexpected errors?

Here is a snippet of code with looging statements for the two queries and the resulting log

CODE:

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("-----------------------------------------------------------")

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
-----------------------------------------------------------

Answer 1

size on an ActiveRecord::Relation object translates to count, because the former tries to get the count of the Relation. But when you call count on a grouped Relation object, you receive a hash.

The keys of this hash are the grouped column's values; the values of this hash are the respective counts.

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

This is true for the following methods: count, sum, average, maximum, and minimum.

If you want to check for rows being present or not, simply use exists? i.e. do the following:

AssessmentResponse.group(:client_id).exists?

Instead of this:

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