Added constraints to services.postage: not nullable and can only containt "first" or "second"

migrations/versions/0227_postage_constraints.py

@@ -0,0 +1,33 @@
+"""
+Revision ID: 0227_postage_constraints
+Revises: 0226_service_postage
+Create Date: 2018-09-13 16:23:59.168877
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = '0227_postage_constraints'
+down_revision = '0226_service_postage'
+
+
+def upgrade():
+    op.execute("""
+            update
+                services
+            set
+                postage = 'second'
+        """)
+
+    op.create_check_constraint(
+        'ck_services_postage',
+        'services',
+        "postage in ('second', 'first')"
+    )
+    op.alter_column('services', 'postage', nullable=False)
+
+
+def downgrade():
+    op.drop_constraint('ck_services_postage', 'services')
+    op.alter_column('services', 'postage',
+                    existing_type=sa.VARCHAR(length=255),
+                    nullable=True)