Fix migration calling Model methods

ee/db/post_migrate/20180713171825_update_epic_dates_from_milestones.rb

@@ -8,6 +8,49 @@ class UpdateEpicDatesFromMilestones < ActiveRecord::Migration
   class Epic < ActiveRecord::Base
     self.table_name = 'epics'
     include EachBatch
+
+    def update_dates
+      milestone_data = fetch_milestone_date_data
+
+      self.start_date = start_date_is_fixed? ? start_date_fixed : milestone_data[:start_date]
+      self.start_date_sourcing_milestone_id = milestone_data[:start_date_sourcing_milestone_id]
+      self.due_date = due_date_is_fixed? ? due_date_fixed : milestone_data[:due_date]
+      self.due_date_sourcing_milestone_id = milestone_data[:due_date_sourcing_milestone_id]
+
+      save if changed?
+    end
+
+    private
+
+    def fetch_milestone_date_data
+      sql = <<~SQL
+        SELECT milestones.id, milestones.start_date, milestones.due_date FROM milestones 
+        INNER JOIN issues ON issues.milestone_id = milestones.id
+        INNER JOIN epic_issues ON epic_issues.issue_id = issues.id
+        INNER JOIN (
+          SELECT MIN(milestones.start_date) AS start_date, MAX(milestones.due_date) AS due_date
+          FROM milestones 
+          INNER JOIN issues ON issues.milestone_id = milestones.id
+          INNER JOIN epic_issues ON epic_issues.issue_id = issues.id
+          WHERE epic_issues.epic_id = #{id}
+        ) inner_results ON (inner_results.start_date = milestones.start_date OR inner_results.due_date = milestones.due_date)
+        WHERE epic_issues.epic_id = #{id}
+        ORDER BY milestones.start_date, milestones.due_date;
+      SQL
+
+      db_results = ActiveRecord::Base.connection.select_all(sql).to_a
+
+      results = {}
+      db_results.find { |row| row['start_date'] }&.tap do |row|
+        results[:start_date] = row['start_date']
+        results[:start_date_sourcing_milestone_id] = row['id']
+      end
+      db_results.reverse.find { |row| row['due_date'] }&.tap do |row|
+        results[:due_date] = row['due_date']
+        results[:due_date_sourcing_milestone_id] = row['id']
+      end
+      results
+    end
   end
 
   def up