Django: Data corrupted after loading? (possible programmer error)

Question

I may be loading data the wrong way.

excerpt of data.json:

{
    "pk": "1",
    "model": "myapp.Course",
    "fields":
    {
        "name": "Introduction to Web Design",
        "requiredFor": [9],
        "offeringSchool": 1,
        "pre_reqs": [],
        "offeredIn": [1, 5, 9]
    }
},

I run python manage.py loaddata -v2 data:

Installed 36 object(s) from 1 fixture(s)

Then, I go to check the above object using the Django shell:

>>> info = Course.objects.filter(id=1)
>>> info.get().pre_reqs.all()
[<Course: Intermediate Web Programming>] # WRONG! There should be no pre-reqs
>>> from django.core import serializers
>>> serializers.serialize("json", info)
'[{"pk": 1, "model": "Apollo.course", "fields": {"pre_reqs": [11], "offeredIn": [1, 5, 9], "offeringSchool": 1, "name": "Introduction to Web Design", "requiredFor": [9]}}]'

The serialized output of the model is not the same as the input that was given to loaddata. The output has a non-empty pre_req list, whereas the input's pre_reqs field is empty. What am I doing wrong?

Answer 1

I think there is already content in your many-to-many table pre_reqs (with FK=1) (before you load your JSON data).

It seems the loader will not delete already existing tuples in many-to-many tables.

Have a look at the django.core.serializer.base.DeserializedObject class. The DeserializedObject.save method only adds new relations.