Add methods to add/update letter contacts for a service that handle defaults properly

app/dao/service_letter_contact_dao.py

@@ -28,7 +28,7 @@ def create_or_update_letter_contact(service_id, contact_block):
         letter_contacts[0].contact_block = contact_block
         dao_update_letter_contact(letter_contacts[0])
     else:
-        # Once we move allowing letter contact blocks, this method will be removed
+        # TODO: Once we move allowing letter contact blocks, this method will be removed
         raise InvalidRequest(
             "Multiple letter contacts were found, this method should not be used.",
             status_code=500
@@ -43,3 +43,65 @@ def dao_create_letter_contact(letter_contact):
 @transactional
 def dao_update_letter_contact(letter_contact):
     db.session.add(letter_contact)
+
+
+@transactional
+def add_letter_contact_for_service(service_id, contact_block, is_default):
+    old_default = _get_existing_default(service_id)
+    if is_default:
+        _reset_old_default_to_false(old_default)
+    else:
+        _raise_when_no_default(old_default)
+
+    new_letter_contact = ServiceLetterContact(
+        service_id=service_id,
+        contact_block=contact_block,
+        is_default=is_default
+    )
+    db.session.add(new_letter_contact)
+    return new_letter_contact
+
+
+@transactional
+def update_letter_contact(service_id, letter_contact_id, contact_block, is_default):
+    old_default = _get_existing_default(service_id)
+    # if we want to make this the default, ensure there are no other existing defaults
+    if is_default:
+        _reset_old_default_to_false(old_default)
+    else:
+        if old_default.id == letter_contact_id:
+            raise InvalidRequest("You must have at least one letter contact as the default.", 400)
+
+    letter_contact_update = ServiceLetterContact.query.get(letter_contact_id)
+    letter_contact_update.contact_block = contact_block
+    letter_contact_update.is_default = is_default
+    db.session.add(letter_contact_update)
+    return letter_contact_update
+
+
+def _get_existing_default(service_id):
+    letter_contacts = dao_get_letter_contacts_by_service_id(service_id=service_id)
+    if letter_contacts:
+        old_default = [x for x in letter_contacts if x.is_default]
+        if len(old_default) == 1:
+            return old_default[0]
+        else:
+            raise Exception(
+                "There should only be one default letter contact for each service. Service {} has {}".format(
+                    service_id,
+                    len(old_default)
+                )
+            )
+    return None
+
+
+def _reset_old_default_to_false(old_default):
+    if old_default:
+        old_default.is_default = False
+        db.session.add(old_default)
+
+
+def _raise_when_no_default(old_default):
+    # check that the update is not updating the only default to false
+    if not old_default:
+        raise InvalidRequest("You must have at least one letter contact as the default.", 400)