Wednesday, February 20, 2013

Generating and Sorting on a Transient Property in a Django Model Class

I ran into an interesting little issue in a Django application today that led to what I thought was some pretty powerful stuff in Python and Django that I hadn't had to use before, so I thought I'd share.

I'm working on an application that to keep it generic I'll simply say deals with requests from users, and these requests require various levels of approval depending on the type and severity of the request.

Here's a basic Django model class that contains the fields relative to this example:

Basic stuff so far.

Where the wrinkle comes in is there is a page in the application that lists the requests ordered by date, but the date by which each request will be displayed and sorted depends upon the impact of the request as follows:
  • minor: use datetime_supervisor_approved
  • major: use datetime_admin_approved
  • severe: use datetime_president_approved
To state the issues succinctly:
  • The date on which I want to sort varies for each request
  • I don't want to store an additional, redundant field in the database simply to have a consistent name to use for sorting
  • Because the sort date isn't stored in the database I can't use order_by on a QuerySet
That last bullet was the killer. In Python it's simple enough to add attributes to a class on the fly, so I could loop over the QuerySet and use conditional logic around the change impact to add a new calendar_date field to each instance of the UserRequest class, but that's kind of ugly because the business logic winds up in a view function as opposed to being in the model, and still doesn't solve the inability to use order_by in the QuerySet.

Because I want to keep the business logic in the model (fat models FTW!), I looked into using a Python property to add a transient attribute to my UserRequest class that is the result of a function call.

Basically what this means in concrete terms is I'm adding a calendar_date attribute to my model class and the value of calendar_date is set by calling a function in the class itself that contains the aforementioned conditional logic around the request impact.

Here's the modified model class:

The big addition here is the _get_calendar_date function that returns a calendar date based on the impact of the request. This is added as an attribute to the class, but it's not saved to the database which is exactly what I wanted in this instance.

That solves one piece of the puzzle, but the other piece is sorting the requests based on this new transient property, which again can't be done by using order_by on the QuerySet since the field on which we want to order isn't in the database.

This is where Python's sorted() function comes in. sorted() can take any iterable (which the Django QuerySet is) and sort it based on a provided comparator function, and this is where I can leverage the transient calendar_date property to sort the requests.

Putting all the pieces together in the view function, here's how it looks:

Python's sorted() function is pretty powerful and straight-forward -- throw it an iterable and what to compare on, and it does the heavy lifting for you.

The only potentially tricky part of this is setting the key since it uses Python's lambda statement (great explanation here), which is Python's way of kinda sorta dipping a toe into the functional programming waters by letting you define an anonymous inline function.

In this case how the lambda plays out is to sort the QuerySet based on two attributes: first the transient calendar_date property that isn't stored in the database, and subsequently the request_impact attribute that is stored in the database. Perfect!

I'm sure none of this is rocket science to seasoned Python and Django veterans but since I hadn't run into the need to do this before I thought I'd document what I came up with both for my own future reference and hopefully for the benefit of others who made need to so something similar.

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