Update postage db constraints for international letters

The `notifications`, `notification_history`, `templates` and `templates_history`
tables all had a check constraint on the postage column which specified
that the postage had to be `first` or `second` if the notification or
template was a letter. We now have two more options for postage -
`europe` and `rest-of-world`.

It's not possible to alter a check constraint, so the constraints have
to be dropped then recreated. We are not recreating the constraint on
the `notification_history` table since values here are always copied
from the `notifications` table.

The constraints get added as `NOT VALID` at first - this stage will lock
the tables, so updating the `notification` table and `templates` and
`templates_history` are done in separate migrations so that we don't lock
all tables at the same time. In a third migration we then run
`VALIDATE CONSTRAINT` for all tables - this will lock a row at a time,
not the whole table.

migrations/versions/0321_update_postage_constraint_1.py

@@ -0,0 +1,83 @@
+"""
+
+Revision ID: 0321_update_postage_constraint_1
+Revises: 0320_optimise_notifications
+Create Date: 2020-05-12 16:17:21.874281
+
+"""
+import os
+
+from alembic import op
+
+
+revision = '0321_update_postage_constraint_1'
+down_revision = '0320_optimise_notifications'
+environment = os.environ['NOTIFY_ENVIRONMENT']
+
+
+def upgrade():
+    op.drop_constraint('chk_notifications_postage_null', 'notifications')
+    op.execute("""
+        ALTER TABLE notifications ADD CONSTRAINT "chk_notifications_postage_null"
+        CHECK (
+            CASE WHEN notification_type = 'letter' THEN
+                postage is not null and postage in ('first', 'second', 'europe', 'rest-of-world')
+            ELSE
+                postage is null
+            END
+        )
+        NOT VALID
+    """)
+    if environment not in ["live", "production"]:
+        op.execute('ALTER TABLE notification_history DROP CONSTRAINT IF EXISTS chk_notification_history_postage_null')
+
+
+def downgrade():
+    pass
+    # To downgrade this migration and migrations 0322 and 0323 * LOCALLY ONLY * use the following code.
+    # This should not be used in production - it will lock the tables for a long time
+    #
+    # op.drop_constraint('chk_notifications_postage_null', 'notifications')
+    # op.drop_constraint('chk_templates_postage', 'templates')
+    # op.drop_constraint('chk_templates_history_postage', 'templates_history')
+    #
+    # op.execute("""
+    #     ALTER TABLE notifications ADD CONSTRAINT "chk_notifications_postage_null"
+    #     CHECK (
+    #         CASE WHEN notification_type = 'letter' THEN
+    #             postage is not null and postage in ('first', 'second')
+    #         ELSE
+    #             postage is null
+    #         END
+    #     )
+    # """)
+    # op.execute("""
+    #     ALTER TABLE notification_history ADD CONSTRAINT "chk_notification_history_postage_null"
+    #     CHECK (
+    #         CASE WHEN notification_type = 'letter' THEN
+    #             postage is not null and postage in ('first', 'second')
+    #         ELSE
+    #             postage is null
+    #         END
+    #     )
+    # """)
+    # op.execute("""
+    #     ALTER TABLE templates ADD CONSTRAINT "chk_templates_postage"
+    #     CHECK (
+    #         CASE WHEN template_type = 'letter' THEN
+    #             postage is not null and postage in ('first', 'second')
+    #         ELSE
+    #             postage is null
+    #         END
+    #     )
+    # """)
+    # op.execute("""
+    #     ALTER TABLE templates_history ADD CONSTRAINT "chk_templates_history_postage"
+    #     CHECK (
+    #         CASE WHEN template_type = 'letter' THEN
+    #             postage is not null and postage in ('first', 'second')
+    #         ELSE
+    #             postage is null
+    #         END
+    #     )
+    # """)

migrations/versions/0322_update_postage_constraint_2.py

@@ -0,0 +1,44 @@
+"""
+
+Revision ID: 0322_update_postage_constraint_2
+Revises: 0321_update_postage_constraint_1
+Create Date: 2020-05-12 16:20:16.548347
+
+"""
+from alembic import op
+
+
+revision = '0322_update_postage_constraint_2'
+down_revision = '0321_update_postage_constraint_1'
+
+
+def upgrade():
+    op.drop_constraint('chk_templates_postage', 'templates')
+    op.drop_constraint('chk_templates_history_postage', 'templates_history')
+
+    op.execute("""
+        ALTER TABLE templates ADD CONSTRAINT "chk_templates_postage"
+        CHECK (
+            CASE WHEN template_type = 'letter' THEN
+                postage is not null and postage in ('first', 'second', 'europe', 'rest-of-world')
+            ELSE
+                postage is null
+            END
+        )
+        NOT VALID
+    """)
+    op.execute("""
+        ALTER TABLE templates_history ADD CONSTRAINT "chk_templates_history_postage"
+        CHECK (
+            CASE WHEN template_type = 'letter' THEN
+                postage is not null and postage in ('first', 'second', 'europe', 'rest-of-world')
+            ELSE
+                postage is null
+            END
+        )
+        NOT VALID
+    """)
+
+
+def downgrade():
+    pass

migrations/versions/0323_update_postage_constraint_3.py

@@ -0,0 +1,22 @@
+"""
+
+Revision ID: 0323_update_postage_constraint_3
+Revises: 0322_update_postage_constraint_2
+Create Date: 2020-05-12 16:21:56.210025
+
+"""
+from alembic import op
+
+
+revision = '0323_update_postage_constraint_3'
+down_revision = '0322_update_postage_constraint_2'
+
+
+def upgrade():
+    op.execute('ALTER TABLE notifications VALIDATE CONSTRAINT "chk_notifications_postage_null"')
+    op.execute('ALTER TABLE templates VALIDATE CONSTRAINT "chk_templates_postage"')
+    op.execute('ALTER TABLE templates_history VALIDATE CONSTRAINT "chk_templates_history_postage"')
+
+
+def downgrade():
+    pass