Get and set ForeignKey directly in Django

Question

I'm trying to work with Django model created from a mysql database which has composite foreign keys.

My models.py goes like this.

class Make(models.Model):
    idmake = models.IntegerField(primary_key=True)
    make = models.CharField(max_length=20L, unique=True, blank=True)
    def __unicode__(self):
      return self.make    
    class Meta:
        db_table = 'make'

class Models(models.Model):
    idmodels = models.IntegerField(primary_key=True, unique=True)
    make = models.ForeignKey(Make, db_column='make', to_field='make')
    model = models.CharField(max_length=45L, unique=True)
    resource_type = models.CharField(max_length=7L)
    def __unicode__(self):
      return self.model    
    class Meta:
        db_table = 'models'

class Systems(models.Model):
    idsystems = models.IntegerField(primary_key=True,  unique=True)
    make = models.ForeignKey(Models, null=True, db_column='make', blank=True, related_name='system_make')
    model = models.ForeignKey(Models, null=True, db_column='model', to_field = 'model', blank=True, related_name='system_model')
    serial_num = models.CharField(max_length=45L, blank=True)
    service_tag = models.CharField(max_length=45L, blank=True)
    mac = models.CharField(max_length=45L, unique=True)

Now when I try to access the make field of Systems I get a ValueError .

>>> s = Systems.objects.get(pk=1)
>>> s.model
<Models: model11>
>>> s.model.make
<Make: make1>
>>> s.make
    Traceback (most recent call last):
    File "<console>", line 1, in <module>
    File "/usr/lib/python2.7/site-packages/django/db/models/fields/related.py", line 384, in __get__
      rel_obj = qs.get(**params)
    File "/usr/lib/python2.7/site-packages/django/db/models/query.py", line 395, in get
      clone = self.filter(*args, **kwargs)
    File "/usr/lib/python2.7/site-packages/django/db/models/query.py", line 669, in filter
      return self._filter_or_exclude(False, *args, **kwargs)
    File "/usr/lib/python2.7/site-packages/django/db/models/query.py", line 687, in _filter_or_exclude
      clone.query.add_q(Q(*args, **kwargs))
    File "/usr/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1271, in add_q
      can_reuse=used_aliases, force_having=force_having)
    File "/usr/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1202, in add_filter 
      connector)
    File "/usr/lib/python2.7/site-packages/django/db/models/sql/where.py", line 71, in add 
      value = obj.prepare(lookup_type, value)
    File "/usr/lib/python2.7/site-packages/django/db/models/sql/where.py", line 339, in prepare
      return self.field.get_prep_lookup(lookup_type, value)
    File "/usr/lib/python2.7/site-packages/django/db/models/fields/__init__.py", line 1003, in get_prep_lookup
      return super(IntegerField, self).get_prep_lookup(lookup_type, value)
    File "/usr/lib/python2.7/site-packages/django/db/models/fields/__init__.py", line 322, in get_prep_lookup
      return self.get_prep_value(value)
    File "/usr/lib/python2.7/site-packages/django/db/models/fields/__init__.py", line 997, in get_prep_value
      return int(value)
    ValueError: invalid literal for int() with base 10: 'make1'

I'm not allowed to change the tables and relations in the database. I'm very new to Django and I'm unable to figure out what is the correct fix for this issue. Basically I would like to be able to get and set the make field of the Systems model directly. Can someone guide me on how I should go about doing this? My initial thoughts were that I would have to create a custom ForeignKey field.

Answer 1

I suspect your codes don't match your DB schema. In this line:

make = models.ForeignKey(Models, null=True, db_column='make', blank=True, related_name='system_make')

Models is supposed to be Make , isn't it? And your Systems.model has to_field = 'model' , did you miss to_field='make' for Systems.make ? I suggest you to drop the whole DB, run syncdb and create test data again. Then see if the error still happen.

Some more tips for your code:

• as your defined to_field = 'model' and to_field='make' , you'd better to consider add db_index=True for make and model fields. Otherwise query performance may be bad when your dataset is large

• if you're going to set Make.make and Models.model to be unique and indexed, they seems to be qualified as primary key. Are the idmake and idmodels really necessary in your case?

• primary_key=True guarantees unique. unique=True is redundant

• Django conversions use singular form for model definition. Ie, use Model System , rather than Models Systems . Also, usually we use id , rather than idmake , idmodels . Those are just conversions, up to you

Answer 2

I discovered the answer, rather by accident. Would like to share for anyone facing similar problem. To access make directly from Systems

>>> s = Systems.objects.get(pk=1)
>>> s.model
<Models: model11>
>>> s.model.make
<Make: make1>
>>> s.make_id
u'make1'

To set make directly from a Systems object

>>> s.make_id = Models.objects.get(model='model21').make.make
>>> s.save()
>>> s.make_id
u'make2'

Be warned. This will not work if the get method of Models returns multiple or no model objects. For example if my Models table was as:

then

>>>> Models.objects.get(model='unknown').make.make
Traceback (most recent call last):
....
MultipleObjectsReturned: get() returned more than one Models -- it returned 2!
>>> Models.objects.get(model='not known').make.make
Traceback (most recent call last):
....
DoesNotExist: Models matching query does not exist.

I fell the programmer needs to be cautious about these things.

EDIT From comments by @ZZY to this answer and to his own answer

class Models(models.Model):
    idmodels = models.IntegerField(primary_key=True, unique=True)
    make = models.ForeignKey(Make, db_column='make', to_field='make')
    model = models.CharField(max_length=45L, unique=True)
    resource_type = models.CharField(max_length=7L)
    def __unicode__(self):
      return self.model    
    class Meta:
        db_table = 'models'
        unique_together = ("make", "model")

And also since model field in Models is unique, the scenario described by me should not occur in a correct table