Create DAO for daily sorted letter model

app/dao/daily_sorted_letter_dao.py

@@ -0,0 +1,37 @@
+from datetime import datetime
+
+from sqlalchemy.dialects.postgresql import insert
+
+from app import db
+from app.dao.dao_utils import transactional
+from app.models import DailySortedLetter
+
+
+def dao_get_daily_sorted_letter_by_billing_day(billing_day):
+    return DailySortedLetter.query.filter_by(
+        billing_day=billing_day
+    ).first()
+
+
+@transactional
+def dao_create_or_update_daily_sorted_letter(new_daily_sorted_letter):
+    '''
+    This uses the Postgres upsert to avoid race conditions when two threads try and insert
+    at the same row. The excluded object refers to values that we tried to insert but were
+    rejected.
+    http://docs.sqlalchemy.org/en/latest/dialects/postgresql.html#insert-on-conflict-upsert
+    '''
+    table = DailySortedLetter.__table__
+    stmt = insert(table).values(
+        billing_day=new_daily_sorted_letter.billing_day,
+        unsorted_count=new_daily_sorted_letter.unsorted_count,
+        sorted_count=new_daily_sorted_letter.sorted_count)
+    stmt = stmt.on_conflict_do_update(
+        index_elements=[table.c.billing_day],
+        set_={
+            'unsorted_count': table.c.unsorted_count + stmt.excluded.unsorted_count,
+            'sorted_count': table.c.sorted_count + stmt.excluded.sorted_count,
+            'updated_at': datetime.utcnow()
+        }
+    )
+    db.session.connection().execute(stmt)