Add dao to store letter contacts and upsert

tests/app/dao/test_service_letter_contact_dao.py

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+import pytest
+
+from app.dao.service_letter_contact_dao import (
+    create_or_update_letter_contact,
+    dao_get_letter_contacts_by_service_id,
+)
+from app.errors import InvalidRequest
+from app.models import ServiceLetterContact
+from tests.app.db import create_letter_contact, create_service
+
+
+def test_dao_get_letter_contacts_by_service_id(notify_db_session):
+    service = create_service()
+    default_letter_contact = create_letter_contact(service=service, contact_block='Edinburgh, ED1 1AA')
+    another_letter_contact = create_letter_contact(service=service, contact_block='Cardiff, CA1 2DB')
+
+    results = dao_get_letter_contacts_by_service_id(service_id=service.id)
+
+    assert len(results) == 2
+    assert default_letter_contact in results
+    assert another_letter_contact in results
+
+
+def test_create_or_update_letter_contact_creates_new_entry(notify_db_session):
+    service = create_service()
+
+    create_or_update_letter_contact(service.id, 'Cardiff, CA1 2DB')
+
+    letter_contacts = dao_get_letter_contacts_by_service_id(service.id)
+
+    assert ServiceLetterContact.query.count() == 1
+    assert letter_contacts[0].service.id == service.id
+    assert letter_contacts[0].contact_block == 'Cardiff, CA1 2DB'
+
+
+def test_create_or_update_letter_contact_does_not_create_another_entry(notify_db_session):
+    service = create_service()
+    create_letter_contact(service, 'London, NW1 2DB')
+    create_or_update_letter_contact(service.id, 'Bristol, BR1 2DB')
+
+    letter_contacts = dao_get_letter_contacts_by_service_id(service.id)
+
+    assert len(letter_contacts) == 1
+
+
+def test_create_or_update_letter_contact_updates_existing_entry(notify_db_session):
+    service = create_service()
+    create_letter_contact(service, 'London, NW1 2DB')
+
+    create_or_update_letter_contact(service.id, 'Bristol, BR1 2DB')
+
+    letter_contact = dao_get_letter_contacts_by_service_id(service.id)
+
+    assert len(letter_contact) == 1
+    assert letter_contact[0].service.id == service.id
+    assert letter_contact[0].contact_block == 'Bristol, BR1 2DB'
+
+
+def test_create_or_update_letter_contact_raises_exception_if_multiple_contact_blocks_exist(notify_db_session):
+    service = create_service()
+    create_letter_contact(service=service, contact_block='Edinburgh, ED1 1AA')
+    create_letter_contact(service=service, contact_block='Manchester, MA1 2BB', is_default=False)
+
+    with pytest.raises(expected_exception=InvalidRequest) as e:
+        create_or_update_letter_contact(service_id=service.id, contact_block='Swansea, SN1 3CC')
+    assert e.value.message == "Multiple letter contacts were found, this method should not be used."