Make postage column part of ft_billing primary key

Now that the postage column is populated and there are no null values,
it can be added to the composite primary key of ft_billing.

app/models.py

@@ -1811,7 +1811,7 @@ class FactBilling(db.Model):
     rate_multiplier = db.Column(db.Integer(), nullable=False, primary_key=True)
     international = db.Column(db.Boolean, nullable=False, primary_key=True)
     rate = db.Column(db.Numeric(), nullable=False, primary_key=True)
-    postage = db.Column(db.String)
+    postage = db.Column(db.String, nullable=False, primary_key=True)
     billable_units = db.Column(db.Integer(), nullable=True)
     notifications_sent = db.Column(db.Integer(), nullable=True)
     created_at = db.Column(db.DateTime, nullable=False, default=datetime.datetime.utcnow)

migrations/versions/0235_add_postage_to_pk.py

@@ -0,0 +1,39 @@
+"""
+
+Revision ID: 0235_add_postage_to_pk
+Revises: 0234_ft_billing_postage
+Create Date: 2018-09-28 15:39:21.115358
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = '0235_add_postage_to_pk'
+down_revision = '0234_ft_billing_postage'
+
+
+def upgrade():
+    op.drop_constraint('ft_billing_pkey', 'ft_billing', type_='primary')
+    op.create_primary_key('ft_billing_pkey', 'ft_billing', ['bst_date',
+                                                            'template_id',
+                                                            'service_id',
+                                                            'notification_type',
+                                                            'provider',
+                                                            'rate_multiplier',
+                                                            'international',
+                                                            'rate',
+                                                            'postage'])
+
+
+def downgrade():
+    op.drop_constraint('ft_billing_pkey', 'ft_billing', type_='primary')
+    op.alter_column('ft_billing', 'postage', nullable=True)
+    op.create_primary_key('ft_billing_pkey', 'ft_billing', ['bst_date',
+                                                            'template_id',
+                                                            'service_id',
+                                                            'notification_type',
+                                                            'provider',
+                                                            'rate_multiplier',
+                                                            'international',
+                                                            'rate'])