Slight perf improvement for fixtures

src/registrar/fixtures_applications.py

@@ -1,7 +1,7 @@
 import logging
 import random
 from faker import Faker
-
+from django.db import transaction
 from registrar.models import (
     User,
     DomainApplication,
@@ -184,6 +184,14 @@ class DomainApplicationFixture:
             logger.warning(e)
             return
 
+        # Lumped under .atomic to ensure we don't make redundant DB calls.
+        # This bundles them all together, and then saves it in a single call.
+        with transaction.atomic():
+            cls._create_applications(users)
+    
+    @classmethod
+    def _create_applications(cls, users):
+        """Creates DomainApplications given a list of users"""
         for user in users:
             logger.debug("Loading domain applications for %s" % user)
             for app in cls.DA:
@@ -211,8 +219,16 @@ class DomainFixture(DomainApplicationFixture):
             logger.warning(e)
             return
 
+        # Lumped under .atomic to ensure we don't make redundant DB calls.
+        # This bundles them all together, and then saves it in a single call.
+        with transaction.atomic():
+            # approve each user associated with `in review` status domains
+            DomainFixture._approve_applications(users)
+
+    @staticmethod
+    def _approve_applications(users):
+        """Approves all provided applications if they are in the state in_review"""
         for user in users:
-            # approve one of each users in review status domains
             application = DomainApplication.objects.filter(
                 creator=user, status=DomainApplication.ApplicationStatus.IN_REVIEW
             ).last()
@@ -220,5 +236,13 @@ class DomainFixture(DomainApplicationFixture):
 
             # We don't want fixtures sending out real emails to
             # fake email addresses, so we just skip that and log it instead
+
+            # All approvals require an investigator, so if there is none,
+            # assign one.
+            if application.investigator is None:
+                # All "users" in fixtures have admin perms per prior config.
+                # No need to check for that.
+                application.investigator = random.choice(users)
+
             application.approve(send_email=False)
             application.save()

src/registrar/fixtures_users.py

@@ -1,5 +1,6 @@
 import logging
 from faker import Faker
+from django.db import transaction
 
 from registrar.models import (
     User,
@@ -186,5 +187,12 @@ class UserFixture:
 
     @classmethod
     def load(cls):
+        # Lumped under .atomic to ensure we don't make redundant DB calls.
+        # This bundles them all together, and then saves it in a single call.
+        # This is slightly different then bulk_create or bulk_update, in that
+        # you still get the same behaviour of .save(), but those incremental 
+        # steps now do not need to close/reopen a db connection, 
+        # instead they share one.
+        with transaction.atomic():
             cls.load_users(cls, cls.ADMINS, "full_access_group")
             cls.load_users(cls, cls.STAFF, "cisa_analysts_group")

src/registrar/models/domain_application.py

@@ -8,6 +8,7 @@ from django.db import models
 from django_fsm import FSMField, transition  # type: ignore
 from django.utils import timezone
 from registrar.models.domain import Domain
+from registrar.utility.errors import ApplicationStatusError, FSMErrorCodes
 
 from .utility.time_stamped_model import TimeStampedModel
 from ..utility.email import send_templated_email, EmailSendingError
@@ -736,8 +737,23 @@ class DomainApplication(TimeStampedModel):
 
         # create the domain
         Domain = apps.get_model("registrar.Domain")
+
+        # == Check that the application is valid == #
         if Domain.objects.filter(name=self.requested_domain.name).exists():
-            raise ValueError("Cannot approve. Requested domain is already in use.")
+            raise ApplicationStatusError(code=FSMErrorCodes.APPROVE_DOMAIN_IN_USE)
+
+        # Check if an investigator is assigned. No approval is possible without one.
+        # TODO - add form level error
+        # TODO - maybe add modal if approver is not investigator as superuser
+        if self.investigator is None:
+            raise ApplicationStatusError(code=FSMErrorCodes.APPROVE_NO_INVESTIGATOR)
+
+        # Investigators must be staff users. 
+        # This is handled elsewhere, but we should check here as a precaution.
+        if not self.investigator.is_staff:
+            raise ApplicationStatusError(code=FSMErrorCodes.APPROVE_INVESTIGATOR_NOT_STAFF)
+
+        # == Create the domain and related components == #
         created_domain = Domain.objects.create(name=self.requested_domain.name)
         self.approved_domain = created_domain
 
@@ -751,6 +767,7 @@ class DomainApplication(TimeStampedModel):
             user=self.creator, domain=created_domain, role=UserDomainRole.Roles.MANAGER
         )
 
+        # == Send out an email == #
         self._send_status_update_email(
             "application approved",
             "emails/status_change_approved.txt",

src/registrar/utility/errors.py

@@ -70,6 +70,51 @@ class GenericError(Exception):
     def get_error_message(self, code=None):
         return self._error_mapping.get(code)
 
+# (Q for reviewers) What should this be called? 
+# Not a fan of this name.
+class FSMErrorCodes(IntEnum):
+    """Used when doing FSM transitions.
+    Overview of generic error codes:
+        - 1 APPROVE_DOMAIN_IN_USE The domain is already in use 
+        - 2 APPROVE_NO_INVESTIGATOR No investigator is assigned when approving
+        - 3 APPROVE_INVESTIGATOR_NOT_STAFF Investigator is a non-staff user
+    """
+    APPROVE_DOMAIN_IN_USE = 1
+    APPROVE_NO_INVESTIGATOR = 2
+    APPROVE_INVESTIGATOR_NOT_STAFF = 3
+
+
+class ApplicationStatusError(Exception):
+    """
+    Used to raise exceptions when doing FSM Transitions.
+    Uses `FSMErrorCodes` as an enum.
+    """
+
+    _error_mapping = {
+        FSMErrorCodes.APPROVE_DOMAIN_IN_USE: (
+            "Cannot approve. Requested domain is already in use."
+        ),
+        FSMErrorCodes.APPROVE_NO_INVESTIGATOR: (
+            "Cannot approve. No investigator was assigned."
+        ),
+        FSMErrorCodes.APPROVE_INVESTIGATOR_NOT_STAFF: (
+            "Cannot approve. Investigator is not a staff user."
+        ),
+    }
+
+    def __init__(self, *args, code=None, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.code = code
+        if self.code in self._error_mapping:
+            self.message = self._error_mapping.get(self.code)
+
+    def __str__(self):
+        return f"{self.message}"
+
+    @classmethod
+    def get_error_message(self, code=None):
+        return self._error_mapping.get(code)
+
 
 class NameserverErrorCodes(IntEnum):
     """Used in the NameserverError class for