New strategy for transaction management.

Introduce a contextmanger function to handle exceptions and nested
transactions. Using the nested_transaction will start a
nested transaction with `db.session.begin_nested`, once the nested
transaction is complete the commit will happen.
`@transactional` has been updated to commit unless in a nested
transaction.

app/dao/annual_billing_dao.py

@@ -1,12 +1,12 @@
 from flask import current_app
 
 from app import db
-from app.dao.dao_utils import nested_transactional, transactional
+from app.dao.dao_utils import transactional
 from app.dao.date_util import get_current_financial_year_start_year
 from app.models import AnnualBilling
 
 
-@nested_transactional
+@transactional
 def dao_create_or_update_annual_billing_for_year(service_id, free_sms_fragment_limit, financial_year_start):
     result = dao_get_free_sms_fragment_limit_for_year(service_id, financial_year_start)
 

app/dao/dao_utils.py

@@ -1,4 +1,5 @@
 import itertools
+from contextlib import contextmanager
 from functools import wraps
 
 from app import db
@@ -10,7 +11,10 @@ def transactional(func):
     def commit_or_rollback(*args, **kwargs):
         try:
             res = func(*args, **kwargs)
-            db.session.commit()
+
+            if not db.session.registry().transaction.nested:
+                db.session.commit()
+
             return res
         except Exception:
             db.session.rollback()
@@ -18,21 +22,18 @@ def transactional(func):
     return commit_or_rollback
 
 
-def nested_transactional(func):
-    # This creates a save point for the nested transaction.
-    # You must manage the commit or rollback from outer most call of the nested of the transactions.
-    @wraps(func)
-    def commit_or_rollback(*args, **kwargs):
-        try:
-            db.session.begin_nested()
-            res = func(*args, **kwargs)
-            db.session.commit()
-            return res
-        except Exception:
-            db.session.rollback()
-            raise
+@contextmanager
+def nested_transaction():
+    try:
+        db.session.begin_nested()
+        yield
+        db.session.commit()
 
-    return commit_or_rollback
+        if not db.session.registry().transaction.nested:
+            db.session.commit()
+    except Exception:
+        db.session.rollback()
+        raise
 
 
 class VersionOptions():

app/dao/organisation_dao.py

@@ -1,12 +1,7 @@
 from sqlalchemy.sql.expression import func
 
 from app import db
-from app.dao.dao_utils import (
-    VersionOptions,
-    nested_transactional,
-    transactional,
-    version_class,
-)
+from app.dao.dao_utils import VersionOptions, transactional, version_class
 from app.models import Domain, Organisation, Service, User
 
 
@@ -110,7 +105,7 @@ def _update_organisation_services(organisation, attribute, only_where_none=True)
         db.session.add(service)
 
 
-@nested_transactional
+@transactional
 @version_class(Service)
 def dao_add_service_to_organisation(service, organisation_id):
     organisation = Organisation.query.filter_by(

app/dao/services_dao.py

@@ -7,12 +7,7 @@ from sqlalchemy.orm import joinedload
 from sqlalchemy.sql.expression import and_, asc, case, func
 
 from app import db
-from app.dao.dao_utils import (
-    VersionOptions,
-    nested_transactional,
-    transactional,
-    version_class,
-)
+from app.dao.dao_utils import VersionOptions, transactional, version_class
 from app.dao.date_util import get_current_financial_year
 from app.dao.email_branding_dao import dao_get_email_branding_by_name
 from app.dao.letter_branding_dao import dao_get_letter_branding_by_name
@@ -289,7 +284,7 @@ def dao_fetch_service_by_id_and_user(service_id, user_id):
     ).one()
 
 
-@nested_transactional
+@transactional
 @version_class(Service)
 def dao_create_service(
     service,