Add February 2021 letter rates

All rates are changing, so we add an end date for the current rates and
insert new rates for every post_class, sheet count and crown status.

migrations/versions/0341_new_letter_rates.py

@@ -0,0 +1,62 @@
+"""
+
+Revision ID: 0341_new_letter_rates
+Revises: 0340_stub_training_broadcasts
+Create Date: 2021-01-27 11:58:21.393227
+
+"""
+import itertools
+import uuid
+from datetime import datetime
+
+from alembic import op
+from sqlalchemy.sql import text
+
+from app.models import LetterRate
+
+
+revision = '0341_new_letter_rates'
+down_revision = '0340_stub_training_broadcasts'
+
+
+CHANGEOVER_DATE = datetime(2021, 2, 1, 0, 0)
+
+
+def get_new_rate(sheet_count, post_class):
+    base_prices = {
+        'second': 34,
+        'first': 64,
+        'europe': 81,
+        'rest-of-world': 81,
+    }
+    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)