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Models
===================

Not adding a max_length to models.CharField
----------------------------------------------
If you do not add a max_length to CharFields your models will not validate.

Solution:

Add a max_length. If you want a field with unlimited length, use a TextField.

Adding null=True on subclasses of CharField
---------------------------------------------

Read `about null <https://docs.djangoproject.com/en/dev/ref/models/fields/#null>`_ to find out why null=True should not be used for CharField.
Often people keep this in mind but still use null=True on fields like EmailField, SlugField etc. Note that these fields are subclasses of
CharField and hence not making them as null applies to these fields too.

Not handling for ModelClass.DoesNotExist and ModelClass.MultipleObjectsReturned
---------------------------------------------------------------------------------

If you are doing a `ModelClass.objects.get(this = that) unless you are doing it
for a unique key, this has a potential to raise `ModelClass.MultipleObjectsReturned`
and `ModelClass.DoesNotExist`. This needs to be handled.

Solution:
If one of the query criteria is on an unique field, put it in a `try: .. except ModelClass.DoesNotExits:..`.
If there criteria has no unique field, handle both `DoesNotExist` and `MultipleObjectsReturned`.
In a view function use django.shortcuts.get_object_or_404(ModelClass, criteria = criteria)
 

Overriding .save()
------------------------
Most of the times you would be using `model_obj.save()` without any parameters,
so you may override `.save` as::

    def save(self):
        ...
        super(ModelClass, self).save()
        
This would work until, `.save` is called with a parameter. For example,
when you use `get_or_create`, `.save` is called with as `.save(force_insert = True)`,
which will fail.

Solution:
Override save as::

    def save(self, *args, **kwargs):
        ...
        super(ModelClass, self).save(*args, **kwargs)
        
Trying qs.get('foo') 
------------------------

Django `.get` and '.filter` takes only keywords argument. So `ModelClass.objects.get(foo)`
gives an error. Similarly creating objects as `ModelClass(foo=foo)` takes only
keyword arguments.

Solution:
Use keyword arguments.

Trying to use an stale object
------------------------------------
What is wrong with code?::

    def foo(req):
      obj =ModelClass.objects.get(pk = 5)
      magic(obj.pk)
      payload{'obj':obj} 
      return render_to_response(template, payload, ..)

Django does not have object identity, hence any change made to the database
within obj within `magic` won't be available in the function.

Solution:
There is no easy solution to it, but you can keep
youself from being bitten by this behaviour. Pull objects close to
their use. Keep track of when an used object is updated and pull them again from
the database if updated.