add postage constraint to notification history

A not valid constraint only checks against new rows, not existing rows.
We can call VALIDATE CONSTRAINT against this new constraint to check
the old rows (which we know are good, having run the command from
74961781). Adding a normal constraint acquires an ACCESS EXCLUSIVE
lock, but validate constraint only needs a SHARE UPDATE EXCLUSIVE lock.

see 9d4b8961 and 0a50993f for more information on marking constraints
as not valid.

migrations/versions/0230_noti_postage_constraint.py

@@ -0,0 +1,44 @@
+"""
+
+Revision ID: 0230_noti_postage_constraint
+Revises: 0229_new_letter_rates
+Create Date: 2018-09-19 11:42:52.229430
+
+"""
+from alembic import op
+
+
+revision = '0230_noti_postage_constraint'
+down_revision = '0229_new_letter_rates'
+
+
+def upgrade():
+    op.execute("""
+        ALTER TABLE notifications ADD CONSTRAINT "chk_notifications_postage_null"
+        CHECK (
+            CASE WHEN notification_type = 'letter' THEN
+                postage in ('first', 'second')
+            ELSE
+                postage is null
+            END
+        )
+        NOT VALID
+    """)
+    op.execute("""
+        ALTER TABLE notification_history ADD CONSTRAINT "chk_notification_history_postage_null"
+        CHECK (
+            CASE WHEN notification_type = 'letter' THEN
+                postage in ('first', 'second')
+            ELSE
+                postage is null
+            END
+        )
+        NOT VALID
+    """)
+    op.execute('ALTER TABLE notifications VALIDATE CONSTRAINT "chk_notifications_postage_null"')
+    op.execute('ALTER TABLE notification_history VALIDATE CONSTRAINT "chk_notification_history_postage_null"')
+
+
+def downgrade():
+    op.drop_constraint('chk_notifications_postage_null', 'notifications', type_='check')
+    op.drop_constraint('chk_notification_history_postage_null', 'notification_history', type_='check')