Remove the course_id property on student.CourseEnrollment.

It conflicts with an underlying related field on that model which seems
to be getting at the same value from the related table.

Add logging for incorrectly instantiating CourseEnrollment models.
This is to catch any places that might break this that are outside of
edx-platform. Django won't accept `course` values that aren't
course_overviews so we don't need extra logic to test that `course`
values are of the correct type.

fixup! Remove the course_id property on student.CourseEnrollment.

common/djangoapps/student/models.py

@@ -13,6 +13,7 @@ file and check it in at the same time as your model changes. To do that,
 
 
 import hashlib
+import inspect
 import json
 import logging
 import uuid
@@ -1094,17 +1095,6 @@ class CourseEnrollment(models.Model):
     def course_price(self):
         return get_cosmetic_verified_display_price(self.course)
 
-    @property
-    def course_id(self):
-        return self._course_id
-
-    @course_id.setter
-    def course_id(self, value):
-        if isinstance(value, six.string_types):
-            self._course_id = CourseKey.from_string(value)
-        else:
-            self._course_id = value
-
     created = models.DateTimeField(auto_now_add=True, null=True, db_index=True)
 
     # If is_active is False, then the student is not considered to be enrolled
@@ -1136,6 +1126,13 @@ class CourseEnrollment(models.Model):
         ordering = ('user', 'course')
 
     def __init__(self, *args, **kwargs):
+        if 'course_id' in kwargs:
+            course_id = kwargs['course_id']
+            if isinstance(course_id, str):
+                kwargs['course_id'] = CourseKey.from_string(course_id)
+                call_location = "\n".join("%30s : %s:%d" % (t[3], t[1], t[2]) for t in inspect.stack()[::-1])
+                log.warning("Forced to coerce course_id in CourseEnrollment instantiation: %s", call_location)
+
         super(CourseEnrollment, self).__init__(*args, **kwargs)
 
         # Private variable for storing course_overview to minimize calls to the database.