Question

I have a strange need, a customer asked me to implement a webapp to support his database. Since I love Django I decided to use it.

The extra need which I have difficult with is as follow: the user should have the extra “power” to create model constraints like checking the range of an IntegerField, all without knowing anything about Python.

So, I have some ideas but I’m not sure what is (if there is) the correct one. Here’s my thoughts:

1. Implement a template-side validation using JavaScript
2. Implement a Python Script and a userfriendly way to use it that writes the models.py file (this sounds terrible)
3. Write some Admin Actions using a raw SQL CHECK constraint, but I still don’t know how

Is this request solvable? Can you give me a tip about the simplest way to do it?

Thanks in advance,

—
Flavio

Answer 1

Yes, it is possible indeed. We will use custom object manager and dynamic Q objects to solve the issue. Here’s an example.

1. Create an appropriate model to store info about constraints. Note that we create ForeginKey to ContentType in order to tie our constraint to a specific model class.

   #your_app/models.py
   from django.contrib.contenttypes.models import ContentType
   from django.contrib.auth.models import User
   
   class PerUserConstraint(models.Model):
       user = models.ForeignKey(User)
       content_type = models.ForeignKey(ContentType)
       field_name = models.CharField(max_length = 255)
       predicate = models.CharField(max_length = 255) #for example, 'contains' etc.
       filter_value = models.CharField(max_length = 255)
   

2. Create a custom manager, register it with your models.

    #your_app/models.py
    from django.db.models.query_utils import Q
   
    class ConstraintedManager(models.Manager):
        def get_constrainted_query(self, user):
            qs = self.model.objects.all()
            content_type = ContentType.objects.get_for_model(self.model.__class__)
            constraints = PerUserConstraint.objects.filter(
                                                 user = user,
                                                 content_type = content_type
            )
            if constraints:
                filters = []
                for constraint in constraints:
                    filters.append(('%s__%s' % (constraint.field_name, constraint.predicate), constraint.value)
                # at this point filters should look like this:
                # [('question__contains', 'dinner'), ('question__contains', 'meal')]
                q_list = [Q(x) for x in filters]
                return qs.filter(reduce(operator.and_, q_list))
                # which is something like 
                # return qs.filter(Q(question__contains = 'dinner') & Q(question__contains = 'meal'))
   
           else:
               return qs     
   
    class YourModel(models.Model):
        #some fields
        ... 
        c = ConstraintedManager()
   

So in your views you will be able to use it like this: YourModel.c.get_constrainted_query(user)

Note that I didn’t test this code, but I hope you’ve grasped the concept.

To override queries inside ModelAdmin, use this and this techniques.