change procedure to add not null constraint to large scores table

There is concern that using change_column to add a NOT NULL constraint
on a large table will lock the table for too long. This uses a
procedure that locks the table for a short period of time to add the
constraint, and then kicks off a validation that will run outside of
the lock.

closes GRADE-668

test plan:
 - The migration runs
 - Check that the NOT NULL constraint is on the table.

Change-Id: Idc11f170768cd4af9820e73950c5cfcc6a3354de
Reviewed-on: https://gerrit.instructure.com/133002
Reviewed-by: Rob Orton <[email protected]>
Reviewed-by: Shahbaz Javeed <[email protected]>
Tested-by: Jenkins
QA-Review: Jeremy Neander <[email protected]>
Product-Review: Keith T. Garner <[email protected]>

db/migrate/20171013185152_add_not_null_constraint_to_scores_course_score.rb

@@ -18,8 +18,22 @@
 
 class AddNotNullConstraintToScoresCourseScore < ActiveRecord::Migration[5.0]
   tag :postdeploy
+  disable_ddl_transaction!
 
-  def change
-    change_column_null :scores, :course_score, false
+  def up
+    self.connection.execute(<<SQL)
+      ALTER TABLE #{Score.quoted_table_name}
+       ADD CONSTRAINT course_score_not_null CHECK (course_score IS NOT NULL) NOT VALID;
+SQL
+
+    self.connection.execute(<<SQL)
+      ALTER TABLE #{Score.quoted_table_name} VALIDATE CONSTRAINT course_score_not_null;
+SQL
+  end
+
+  def down
+    self.connection.execute(<<SQL)
+      ALTER TABLE #{Score.quoted_table_name} DROP CONSTRAINT course_score_not_null;
+SQL
   end
 end