Skip to main content

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
            # 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 
                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


Post a Comment

Popular posts from this blog

Django Docker and Celery

I've finally had the time to create a Django+Celery project that can be completely run using Docker and Docker Compose. I'd like to share some of the steps that helped me achieve this. I've created an example project that I've used to demo this process. The example project can be viewed here on Github.

To run this example, you will need to have a recent version of Docker and Docker Compose installed. I'm using Docker 17.03.1 and Docker Compose 1.11.2.

Let's take a look at one of the core files, the docker-compose.yml.   You'll notice that I have defined the following services:
db - the service running the Postgres database container, needed for the Django apprabbitmq - service running the RabbitMQ container, needed for queuing jobs submitted by Celeryapp - the service containing Django app containerworker - the service that runs the Celery worker containerweb - the service that runs the Nginx con…

Django Admin Override Save for Model

Sometimes it's nice to be able to add custom code to the save method of objects in the Django Admin.  So, when editing an object on the Admin object detail page (change form), adding the following method override to your ModelAdmin in will allow you to add custom code to the save function.

class MyModelAdmin(admin.ModelAdmin): def save_model(self, request, obj, form, change): # custom stuff here
This is documented in the Django Docs, but I found it particularly useful.

Django: Using Caching to Track Online Users

Recently I wanted a simple solution to track whether a user is online on a given Django site.  The definition of "online" on a site is kind of ambiguous, so I'll define that a user is considered to be online if they have made any request to the site in the last five minutes.

I found that one approach is to use Django's caching framework to track when a user last accessed the site.  For example, upon each request, I can have a middleware set the current time as a cache value associated with a given user.  This allows us to store some basic information about logged-in user's online state without having to hit the database on each request and easily retrieve it by accessing the cache.

My approach below.  Comments welcome.

# add the middleware that you are about to create to settings MIDDLEWARE_CLASSES = ( .... 'middleware.activeuser_middleware.ActiveUserMiddleware', .... ) # Setup caching per Django docs. In actuality, you'd p…