flows: Correctly check initial policies on flow with context

# Conflicts:
#	passbook/flows/planner.py
#	passbook/flows/tests/test_planner.py
#	passbook/flows/tests/test_views.py
#	passbook/flows/views.py

passbook/flows/models.py

@@ -3,12 +3,16 @@ from typing import Optional
 from uuid import uuid4
 
 from django.db import models
+from django.http import HttpRequest
 from django.utils.translation import gettext_lazy as _
 from model_utils.managers import InheritanceManager
+from structlog import get_logger
 
 from passbook.core.types import UIUserSettings
 from passbook.policies.models import PolicyBindingModel
 
+LOGGER = get_logger()
+
 
 class FlowDesignation(models.TextChoices):
     """Designation of what a Flow should be used for. At a later point, this
@@ -62,10 +66,29 @@ class Flow(PolicyBindingModel):
         PolicyBindingModel, parent_link=True, on_delete=models.CASCADE, related_name="+"
     )
 
-    def related_flow(self, designation: str) -> Optional["Flow"]:
+    @staticmethod
+    def with_policy(request: HttpRequest, **flow_filter) -> Optional["Flow"]:
+        """Get a Flow by `**flow_filter` and check if the request from `request` can access it."""
+        from passbook.policies.engine import PolicyEngine
+
+        flows = Flow.objects.filter(**flow_filter)
+        for flow in flows:
+            engine = PolicyEngine(flow, request.user, request)
+            engine.build()
+            result = engine.result
+            if result.passing:
+                LOGGER.debug("with_policy: flow passing", flow=flow)
+                return flow
+            LOGGER.warning(
+                "with_policy: flow not passing", flow=flow, messages=result.messages
+            )
+        LOGGER.debug("with_policy: no flow found", filters=flow_filter)
+        return None
+
+    def related_flow(self, designation: str, request: HttpRequest) -> Optional["Flow"]:
         """Get a related flow with `designation`. Currently this only queries
         Flows by `designation`, but will eventually use `self` for related lookups."""
-        return Flow.objects.filter(designation=designation).first()
+        return Flow.with_policy(request, designation=designation)
 
     def __str__(self) -> str:
         return f"Flow {self.name} ({self.slug})"