Wednesday, April 13, 2011

Django Pattern for Reporting Errors/Messages in Views Part 2: Managers

In my last post, I discussed how you can use a simple UserMessage class in your views to report simple error messages.   This way, you can keep track of errors and report them to the users in a clean fashion.

One thing I didn't expand upon much is how to create some manager methods to cleanly lookup model instances and report any errors to the UserMessage class.  I think this approach is a little nicer than catching exceptions all over the place, because you don't have to clutter your views with try/catch blocks.

In the example below, we create a custom manager that looks up a model instance of "MyModel" with the given hash.  If the selected instance of "MyModel" does not exist or if some other error occurs, it writes an error message to "message" instance passed into it.  You can customize this as you see fit.  For example, in the code below, I pass in a "user" instance that can be used to check permissions in this method. 

class MyModelManager(models.Manager):
    # In this case, we pass in a user, a unique hash for a 
    # "MyModel" object instance, and an optional message. 
    def get_mymodel(self, user, mymodel_hash, message=None):
        mymodel = []
        # If the message previously set an error, return immediately  
        if message:
            return None, message
        try:
            # if the user is an admin, search all MyModels for the matching hash 
            if user.is_staff:
                mymodel = MyModel.objects.get(hash=mymodel_hash, enabled=True)
            # if the user is NOT an admin, search user-related MyModels for the matching hash 
            else:
                mymodel = user.mymodel_set.get(hash=mymodel_hash, enabled=True)
        except MyModel.DoesNotExist:
            message = UserMessage()
            message.title = _("Not found")
            message.text = _("MyModel does not exist")
        return mymodel, message

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