CASCADE fixes in data model

core/models/troggle.py

@@ -11,7 +11,7 @@ import settings
 """This file declares TroggleModel which inherits from django.db.models.Model 
 All TroggleModel and models.Model subclasses inherit persistence in the django relational database. This is known as
 the django Object Relational Mapping (ORM).
-There are more subclasses define in models_caves.py models_survex.py etc.
+There are more subclasses defined in models/caves.py models/survex.py etc.
 """
 
 
@@ -33,7 +33,7 @@ class TroggleModel(models.Model):
 
 class DataIssue(TroggleModel):
     """When importing cave data any validation problems produce a message which is
-    recorded as a DataIssue. The django admin system automatically prodiuces a page listing
+    recorded as a DataIssue. The django admin system automatically produces a page listing
     these at /admin/core/dataissue/
     This is a use of the NOTIFICATION pattern:
     https://martinfowler.com/eaaDev/Notification.html
@@ -55,7 +55,6 @@ class DataIssue(TroggleModel):
     def __str__(self):
         return f"{self.parser} - {self.message}"
 
-
 #
 # single Expedition, usually seen by year
 #
@@ -150,7 +149,10 @@ class Person(TroggleModel):
 
 
 class PersonExpedition(TroggleModel):
-    """Person's attendance to one Expo"""
+    """Person's attendance to one Expo
+    CASCADE means that if an expedition or a person is deleted, the PersonExpedition
+    is deleted too
+    """
 
     expedition = models.ForeignKey(Expedition, on_delete=models.CASCADE)
     person = models.ForeignKey(Person, on_delete=models.CASCADE)