Explicitly set on_delete for ForeignKey fields

Running with PYTHONWARNINGS=all reveals several deprecation warnings
that `on_delete` must be specified for `ForeignKey` fields:

```
RemovedInDjango20Warning: on_delete will be a required arg for
ForeignKey in Django 2.0. Set it to models.CASCADE on models and in
existing migrations if you want to maintain the current default
behavior. See https://docs.djangoproject.com/en/1.11/ref/models/fields/#django.db.models.ForeignKey.on_delete
```

kolibri/core/analytics/models.py

@@ -24,8 +24,8 @@ class PingbackNotificationDismissed(models.Model):
 
     permissions = IsOwn()
 
-    user = models.ForeignKey(FacilityUser)
-    notification = models.ForeignKey(PingbackNotification)
+    user = models.ForeignKey(FacilityUser, on_delete=models.CASCADE)
+    notification = models.ForeignKey(PingbackNotification, on_delete=models.CASCADE)
 
     class Meta:
         unique_together = (("user", "notification"),)

kolibri/core/auth/models.py

@@ -1034,7 +1034,12 @@ class Collection(AbstractFacilityDataModel):
 
     name = models.CharField(max_length=100)
     parent = models.ForeignKey(
-        "self", null=True, blank=True, related_name="children", db_index=True
+        "self",
+        null=True,
+        blank=True,
+        related_name="children",
+        db_index=True,
+        on_delete=models.CASCADE,
     )
     kind = models.CharField(max_length=20, choices=collection_kinds.choices)
 
@@ -1225,11 +1230,15 @@ class Membership(AbstractFacilityDataModel):
     permissions = own | role | membership
 
     user = models.ForeignKey(
-        "FacilityUser", related_name="memberships", blank=False, null=False
+        "FacilityUser",
+        related_name="memberships",
+        blank=False,
+        null=False,
+        on_delete=models.CASCADE,
     )
     # Note: "It's recommended you use mptt.fields.TreeForeignKey wherever you have a foreign key to an MPTT model.
     # https://django-mptt.github.io/django-mptt/models.html#treeforeignkey-treeonetoonefield-treemanytomanyfield
-    collection = TreeForeignKey("Collection")
+    collection = TreeForeignKey("Collection", on_delete=models.CASCADE)
 
     class Meta:
         unique_together = (("user", "collection"),)
@@ -1320,7 +1329,7 @@ class Role(AbstractFacilityDataModel):
     )
     # Note: "It's recommended you use mptt.fields.TreeForeignKey wherever you have a foreign key to an MPTT model.
     # https://django-mptt.github.io/django-mptt/models.html#treeforeignkey-treeonetoonefield-treemanytomanyfield
-    collection = TreeForeignKey("Collection")
+    collection = TreeForeignKey("Collection", on_delete=models.CASCADE)
     kind = models.CharField(max_length=26, choices=role_kinds.choices)
 
     class Meta:

kolibri/core/bookmarks/models.py

@@ -15,7 +15,7 @@ class Bookmark(AbstractFacilityDataModel):
     content_id = UUIDField(blank=True, null=True)
     channel_id = UUIDField(blank=True, null=True)
     contentnode_id = UUIDField()
-    user = models.ForeignKey(FacilityUser, blank=False)
+    user = models.ForeignKey(FacilityUser, blank=False, on_delete=models.CASCADE)
     created = models.DateTimeField(default=timezone.now, db_index=True)
 
     morango_model_name = "bookmark"

kolibri/core/content/base_models.py

@@ -99,7 +99,12 @@ class ContentNode(MPTTModel):
 
     id = UUIDField(primary_key=True)
     parent = TreeForeignKey(
-        "self", null=True, blank=True, related_name="children", db_index=True
+        "self",
+        null=True,
+        blank=True,
+        related_name="children",
+        db_index=True,
+        on_delete=models.CASCADE,
     )
     license_name = models.CharField(max_length=50, null=True, blank=True)
     license_description = models.TextField(null=True, blank=True)
@@ -127,7 +132,9 @@ class ContentNode(MPTTModel):
     author = models.CharField(max_length=200, blank=True)
     kind = models.CharField(max_length=200, choices=content_kinds.choices, blank=True)
     available = models.BooleanField(default=False)
-    lang = models.ForeignKey("Language", blank=True, null=True)
+    lang = models.ForeignKey(
+        "Language", blank=True, null=True, on_delete=models.CASCADE
+    )
 
     # A JSON Dictionary of properties to configure loading, rendering, etc. the file
     options = JSONField(default={}, blank=True, null=True)
@@ -169,12 +176,18 @@ class File(models.Model):
 
     id = UUIDField(primary_key=True)
     # The foreign key mapping happens here as many File objects can map onto a single local file
-    local_file = models.ForeignKey("LocalFile", related_name="files")
-    contentnode = models.ForeignKey("ContentNode", related_name="files")
+    local_file = models.ForeignKey(
+        "LocalFile", related_name="files", on_delete=models.CASCADE
+    )
+    contentnode = models.ForeignKey(
+        "ContentNode", related_name="files", on_delete=models.CASCADE
+    )
     preset = models.CharField(
         max_length=150, choices=format_presets.choices, blank=True
     )
-    lang = models.ForeignKey("Language", blank=True, null=True)
+    lang = models.ForeignKey(
+        "Language", blank=True, null=True, on_delete=models.CASCADE
+    )
     supplementary = models.BooleanField(default=False)
     thumbnail = models.BooleanField(default=False)
     priority = models.IntegerField(blank=True, null=True, db_index=True)
@@ -209,7 +222,9 @@ class AssessmentMetaData(models.Model):
     """
 
     id = UUIDField(primary_key=True)
-    contentnode = models.ForeignKey("ContentNode", related_name="assessmentmetadata")
+    contentnode = models.ForeignKey(
+        "ContentNode", related_name="assessmentmetadata", on_delete=models.CASCADE
+    )
     # A JSON blob containing a serialized list of ids for questions that the assessment can present.
     assessment_item_ids = JSONField(default=[])
     # Length of the above assessment_item_ids for a convenience lookup.
@@ -241,7 +256,7 @@ class ChannelMetadata(models.Model):
     last_updated = DateTimeTzField(null=True)
     # Minimum version of Kolibri that this content database is compatible with
     min_schema_version = models.CharField(max_length=50)
-    root = models.ForeignKey("ContentNode")
+    root = models.ForeignKey("ContentNode", on_delete=models.CASCADE)
 
     class Meta:
         abstract = True

kolibri/core/content/models.py

@@ -468,7 +468,9 @@ class ContentRequest(models.Model):
     """
 
     id = UUIDField(primary_key=True, default=_hex_uuid_str)
-    facility = models.ForeignKey(Facility, related_name="content_requests")
+    facility = models.ForeignKey(
+        Facility, related_name="content_requests", on_delete=models.CASCADE
+    )
 
     # the source model's `morango_model_name` that initiated the request:
     # - for user-initiated requests it should be `facilityuser`

kolibri/core/discovery/models.py

@@ -277,7 +277,7 @@ def allow_migrate(self, db, app_label, model_name=None, **hints):
 class PinnedDevice(models.Model):
 
     instance_id = UUIDField(blank=False)
-    user = models.ForeignKey(FacilityUser, blank=False)
+    user = models.ForeignKey(FacilityUser, blank=False, on_delete=models.CASCADE)
     created = models.DateTimeField(default=timezone.now, db_index=True)
 
     permissions = IsOwn()

kolibri/core/exams/models.py

@@ -112,10 +112,18 @@ class Exam(AbstractFacilityDataModel):
     # who creates them in the context of their class, this stores that relationship but does
     # not assign exam itself to the class - for that see the ExamAssignment model.
     collection = models.ForeignKey(
-        Collection, related_name="exams", blank=False, null=False
+        Collection,
+        related_name="exams",
+        blank=False,
+        null=False,
+        on_delete=models.CASCADE,
     )
     creator = models.ForeignKey(
-        FacilityUser, related_name="exams", blank=False, null=True
+        FacilityUser,
+        related_name="exams",
+        blank=False,
+        null=True,
+        on_delete=models.CASCADE,
     )
 
     # To be set True when the quiz is first set to active=True
@@ -206,12 +214,26 @@ class ExamAssignment(AbstractFacilityDataModel):
         )
         | UserCanReadExamAssignmentData()
     )
-    exam = models.ForeignKey(Exam, related_name="assignments", blank=False, null=False)
+    exam = models.ForeignKey(
+        Exam,
+        related_name="assignments",
+        blank=False,
+        null=False,
+        on_delete=models.CASCADE,
+    )
     collection = models.ForeignKey(
-        Collection, related_name="assigned_exams", blank=False, null=False
+        Collection,
+        related_name="assigned_exams",
+        blank=False,
+        null=False,
+        on_delete=models.CASCADE,
     )
     assigned_by = models.ForeignKey(
-        FacilityUser, related_name="assigned_exams", blank=False, null=True
+        FacilityUser,
+        related_name="assigned_exams",
+        blank=False,
+        null=True,
+        on_delete=models.CASCADE,
     )
 
     def pre_save(self):
@@ -282,8 +304,8 @@ class IndividualSyncableExam(AbstractFacilityDataModel):
 
     morango_model_name = "individualsyncableexam"
 
-    user = models.ForeignKey(FacilityUser)
-    collection = models.ForeignKey(Collection)
+    user = models.ForeignKey(FacilityUser, on_delete=models.CASCADE)
+    collection = models.ForeignKey(Collection, on_delete=models.CASCADE)
     exam_id = models.UUIDField()
 
     serialized_exam = JSONField()

kolibri/core/lessons/models.py

@@ -57,11 +57,19 @@ class Lesson(AbstractFacilityDataModel):
 
     # The Classroom-type Collection for which the Lesson is created
     collection = models.ForeignKey(
-        Collection, related_name="lessons", blank=False, null=False
+        Collection,
+        related_name="lessons",
+        blank=False,
+        null=False,
+        on_delete=models.CASCADE,
     )
 
     created_by = models.ForeignKey(
-        FacilityUser, related_name="lessons_created", blank=False, null=True
+        FacilityUser,
+        related_name="lessons_created",
+        blank=False,
+        null=True,
+        on_delete=models.CASCADE,
     )
     date_created = DateTimeTzField(default=local_now, editable=False)
 
@@ -130,13 +138,25 @@ class LessonAssignment(AbstractFacilityDataModel):
     )
 
     lesson = models.ForeignKey(
-        Lesson, related_name="lesson_assignments", blank=False, null=False
+        Lesson,
+        related_name="lesson_assignments",
+        blank=False,
+        null=False,
+        on_delete=models.CASCADE,
     )
     collection = models.ForeignKey(
-        Collection, related_name="assigned_lessons", blank=False, null=False
+        Collection,
+        related_name="assigned_lessons",
+        blank=False,
+        null=False,
+        on_delete=models.CASCADE,
     )
     assigned_by = models.ForeignKey(
-        FacilityUser, related_name="assigned_lessons", blank=False, null=True
+        FacilityUser,
+        related_name="assigned_lessons",
+        blank=False,
+        null=True,
+        on_delete=models.CASCADE,
     )
 
     def __str__(self):
@@ -214,8 +234,8 @@ class IndividualSyncableLesson(AbstractFacilityDataModel):
 
     morango_model_name = "individualsyncablelesson"
 
-    user = models.ForeignKey(FacilityUser)
-    collection = models.ForeignKey(Collection)
+    user = models.ForeignKey(FacilityUser, on_delete=models.CASCADE)
+    collection = models.ForeignKey(Collection, on_delete=models.CASCADE)
     lesson_id = models.UUIDField()
 
     serialized_lesson = JSONField()

kolibri/core/logger/models.py

@@ -110,7 +110,9 @@ class ContentSessionLog(BaseLogModel):
     # Morango syncing settings
     morango_model_name = "contentsessionlog"
 
-    user = models.ForeignKey(FacilityUser, blank=True, null=True)
+    user = models.ForeignKey(
+        FacilityUser, blank=True, null=True, on_delete=models.CASCADE
+    )
     content_id = UUIDField(db_index=True)
     visitor_id = models.UUIDField(blank=True, null=True)
     channel_id = UUIDField(blank=True, null=True)
@@ -139,7 +141,7 @@ class ContentSummaryLog(BaseLogModel):
     # Morango syncing settings
     morango_model_name = "contentsummarylog"
 
-    user = models.ForeignKey(FacilityUser)
+    user = models.ForeignKey(FacilityUser, on_delete=models.CASCADE)
     content_id = UUIDField(db_index=True)
     channel_id = UUIDField(blank=True, null=True)
     start_timestamp = DateTimeTzField()
@@ -172,7 +174,7 @@ class UserSessionLog(BaseLogModel):
     # Morango syncing settings
     morango_model_name = "usersessionlog"
 
-    user = models.ForeignKey(FacilityUser)
+    user = models.ForeignKey(FacilityUser, on_delete=models.CASCADE)
     channels = models.TextField(blank=True)
     start_timestamp = DateTimeTzField(default=local_now)
     last_interaction_timestamp = DateTimeTzField(null=True, blank=True)
@@ -226,9 +228,11 @@ class MasteryLog(BaseLogModel):
     # Morango syncing settings
     morango_model_name = "masterylog"
 
-    user = models.ForeignKey(FacilityUser)
+    user = models.ForeignKey(FacilityUser, on_delete=models.CASCADE)
     # Every MasteryLog is related to the single summary log for the user/content pair
-    summarylog = models.ForeignKey(ContentSummaryLog, related_name="masterylogs")
+    summarylog = models.ForeignKey(
+        ContentSummaryLog, related_name="masterylogs", on_delete=models.CASCADE
+    )
     # The MasteryLog records the mastery criterion that has been specified for the user.
     # It is recorded here to prevent this changing in the middle of a user's engagement
     # with an assessment.
@@ -288,7 +292,9 @@ class BaseAttemptLog(BaseLogModel):
     # A JSON Array with a sequence of JSON objects that describe the history of interaction of the user
     # with this assessment item in this attempt.
     interaction_history = JSONField(default=[], blank=True)
-    user = models.ForeignKey(FacilityUser, blank=True, null=True)
+    user = models.ForeignKey(
+        FacilityUser, blank=True, null=True, on_delete=models.CASCADE
+    )
     error = models.BooleanField(default=False)
 
     class Meta:
@@ -305,9 +311,15 @@ class AttemptLog(BaseAttemptLog):
 
     # Which mastery log was this attemptlog associated with?
     masterylog = models.ForeignKey(
-        MasteryLog, related_name="attemptlogs", blank=True, null=True
+        MasteryLog,
+        related_name="attemptlogs",
+        blank=True,
+        null=True,
+        on_delete=models.CASCADE,
+    )
+    sessionlog = models.ForeignKey(
+        ContentSessionLog, related_name="attemptlogs", on_delete=models.CASCADE
     )
-    sessionlog = models.ForeignKey(ContentSessionLog, related_name="attemptlogs")
 
     def infer_dataset(self, *args, **kwargs):
         return self.cached_related_dataset_lookup("sessionlog")
@@ -322,9 +334,11 @@ class ExamLog(BaseLogModel):
     morango_model_name = "examlog"
 
     # Identifies the exam that this is for.
-    exam = models.ForeignKey(Exam, related_name="examlogs", blank=False, null=False)
+    exam = models.ForeignKey(
+        Exam, related_name="examlogs", blank=False, null=False, on_delete=models.CASCADE
+    )
     # Identifies which user this log summarizes interactions for.
-    user = models.ForeignKey(FacilityUser)
+    user = models.ForeignKey(FacilityUser, on_delete=models.CASCADE)
     # Is this exam open for engagement, or is it closed?
     # Used to end user engagement with an exam when it has been deactivated.
     closed = models.BooleanField(default=False)
@@ -346,7 +360,11 @@ class ExamAttemptLog(BaseAttemptLog):
     morango_model_name = "examattemptlog"
 
     examlog = models.ForeignKey(
-        ExamLog, related_name="attemptlogs", blank=False, null=False
+        ExamLog,
+        related_name="attemptlogs",
+        blank=False,
+        null=False,
+        on_delete=models.CASCADE,
     )
     # We have no session logs associated with ExamLogs, so we need to record the channel and content
     # ids here