Drop postage constraints

The constaints on notifications and notification_history have already
been dropped in production, but still exist in staging and in dev
environments.

The constraints on templates and templates_history exist in all
environments.

migrations/versions/0321_drop_postage_constraints.py

@@ -0,0 +1,69 @@
+"""
+
+Revision ID: 0321_drop_postage_constraints
+Revises: 0320_optimise_notifications
+Create Date: 2020-06-08 11:48:53.315768
+
+"""
+import os
+
+from alembic import op
+
+
+revision = '0321_drop_postage_constraints'
+down_revision = '0320_optimise_notifications'
+environment = os.environ['NOTIFY_ENVIRONMENT']
+
+
+def upgrade():
+    if environment not in ["live", "production"]:
+        op.execute('ALTER TABLE notifications DROP CONSTRAINT IF EXISTS chk_notifications_postage_null')
+        op.execute('ALTER TABLE notification_history DROP CONSTRAINT IF EXISTS chk_notification_history_postage_null')
+
+    op.execute('ALTER TABLE templates DROP CONSTRAINT IF EXISTS chk_templates_postage')
+    op.execute('ALTER TABLE templates_history DROP CONSTRAINT IF EXISTS chk_templates_history_postage')
+
+
+def downgrade():
+    # The downgrade command must not be run in production - it will lock the tables for a long time
+    if environment not in ["live", "production"]:
+        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
+            )
+        """)