Add new letter rates for March 1, 2022.

- second class postage will go up by 2 pence, plus VAT
- international postage will go up by 7 pence, plus VAT
- first class postage will go down by 6 pence, plus VAT

migrations/versions/0366_letter_rates_2022.py

@@ -0,0 +1,63 @@
+"""
+
+Revision ID: 0366_letter_rates_2022
+Revises: 0365_add_nhs_branding
+Create Date: 2022-03-01 14:00:00
+
+"""
+import itertools
+import uuid
+from datetime import datetime
+
+from alembic import op
+from sqlalchemy.sql import text
+
+from app.models import LetterRate
+
+
+revision = '0366_letter_rates_2022'
+down_revision = '0365_add_nhs_branding'
+
+
+CHANGEOVER_DATE = datetime(2022, 3, 1, 0, 0)
+
+
+def get_new_rate(sheet_count, post_class):
+    base_prices = {
+        'second': 36,
+        'first': 58,
+        'europe': 88,
+        'rest-of-world': 88,
+    }
+    multiplier = 5 if post_class in ('first', 'second') else 8
+
+    return (base_prices[post_class] + (multiplier * sheet_count)) / 100.0
+
+
+def upgrade():
+    conn = op.get_bind()
+    conn.execute(text("UPDATE letter_rates SET end_date = :start WHERE end_date IS NULL"), start=CHANGEOVER_DATE)
+
+    op.bulk_insert(LetterRate.__table__, [
+        {
+            'id': uuid.uuid4(),
+            'start_date': CHANGEOVER_DATE,
+            'end_date': None,
+            'sheet_count': sheet_count,
+            'rate': get_new_rate(sheet_count, post_class),
+            'crown': crown,
+            'post_class': post_class,
+        }
+        for sheet_count, crown, post_class in itertools.product(
+            range(1, 6),
+            [True, False],
+            ['first', 'second', 'europe', 'rest-of-world']
+        )
+    ])
+
+
+def downgrade():
+    # Make sure you've thought about billing implications etc before downgrading!
+    conn = op.get_bind()
+    conn.execute(text("DELETE FROM letter_rates WHERE start_date = :start"), start=CHANGEOVER_DATE)
+    conn.execute(text("UPDATE letter_rates SET end_date = NULL WHERE end_date = :start"), start=CHANGEOVER_DATE)