perf: reduce time spent in sql query for student response rate

app/models/student_response_rate_calculator.rb

@@ -42,12 +42,13 @@ class StudentResponseRateCalculator < ResponseRateCalculator
       threshold = 10
       quarter_of_grade = enrollment_by_grade[grade] / 4
       threshold = threshold > quarter_of_grade ? quarter_of_grade : threshold
-      memo[grade] = SurveyItem.joins("inner join survey_item_responses on  survey_item_responses.survey_item_id = survey_items.id")
-                              .student_survey_items
-                              .where("survey_item_responses.school": school, "survey_item_responses.academic_year": academic_year, "survey_item_responses.grade": grade, "survey_item_responses.survey_item_id": subcategory.survey_items.student_survey_items)
-                              .group("survey_items.id")
-                              .having("count(*) >= #{threshold}")
-                              .count
+
+      si = SurveyItemResponse.student_survey_items_with_sufficient_responses_by_grade(school:, academic_year:,
+                                                                                      threshold:)
+      ssi = @subcategory.survey_items.student_survey_items.map(&:id)
+      grade_array = Array.new(ssi.length, grade)
+
+      memo[grade] = si.slice(*grade_array.zip(ssi))
     end
     @survey_items_with_sufficient_responses[grade]
   end

app/models/survey_item_response.rb

@@ -69,4 +69,18 @@ class SurveyItemResponse < ActiveRecord::Base
     end
     @teacher_survey_items_with_sufficient_responses[[school, academic_year]]
   end
+
+  def self.student_survey_items_with_sufficient_responses_by_grade(school:, academic_year:, threshold:)
+    @student_survey_items_with_sufficient_responses_by_grade ||= Hash.new do |memo, (school, academic_year)|
+      hash = SurveyItem.joins("inner join survey_item_responses on  survey_item_responses.survey_item_id = survey_items.id")
+                       .student_survey_items
+                       .where("survey_item_responses.school": school, "survey_item_responses.academic_year": academic_year)
+                       .group(:grade, :id)
+                       .having("count(*) >= #{threshold}")
+                       .count
+      memo[[school, academic_year]] = hash
+    end
+
+    @student_survey_items_with_sufficient_responses_by_grade[[school, academic_year]]
+  end
 end