enh(models): Add unique_job_executions to ensure uniqueness of `Dja…

…ngoJobExecution`s.

django_apscheduler/models.py

@@ -1,6 +1,7 @@
 from datetime import timedelta, datetime
 
 from django.db import models, transaction
+from django.db.models import UniqueConstraint
 from django.utils import timezone
 from django.utils.translation import gettext_lazy as _
 
@@ -213,3 +214,8 @@ def __str__(self):
 
     class Meta:
         ordering = ("-run_time",)
+        constraints = [
+            UniqueConstraint(
+                fields=["job_id", "run_time"], name="unique_job_executions"
+            )
+        ]

docs/changelog.md

@@ -7,6 +7,9 @@ This changelog is used to track all major changes to django-apscheduler.
 **Fixes**
 
 - Also handle `db.InterfaceError` when retrying database-related operations (thanks @zmmfsj-z).
+- Add `unique_job_executions` constraint to ensure that only one `DjangoJobExecution` can be created for
+  each `DjangoJob`
+  for a specific run time.
 
 ## v0.6.0 (2021-06-17)