Creating a Django model instance in the past

In Django, if you want to save the date an instance was created and/or updated, you can use DateTimeField with the flags auto_now_add or auto_now. They automatically set the field to now every time the object is created and saved, respectively. And it is common to have tasks that depends on the date a specific object was created/updated.

For example, I have an Invoice model that has a paid flag to indicate if it was paid or not and a updated_at field that tells me the last time that invoice was updated, using auto_now flag (every time an invoice is saved, the updated_at field is updated with the current time):

To make easier to control the invoices not paid, I created a task that runs everyday to notify me of all invoices that weren't paid and weren't updated in the past two days:

Really simple, right? Now it is time to test check_pending_invoices task (sure you always have every line of your app covered by tests :). This is a really simple example, so we just need to create two tests to cover the task. The first one should check if the notify function wasn't called, since no invoice was created. To do that, I am going to use mock.patch() decorator from the Python built-in library unittest. The best definition of mocking a function I found is from Mike Lin:

A mock function call returns a predefined value immediately, without doing any work. A mock object’s attributes and methods are similarly defined entirely in the test, without creating the real object or doing any work. The fact that the writer of the test can define the return values of each function call gives him or her a tremendous amount of power when testing […]

So, I'm not actually interest in how notify works or what it returns. The test target is check_pending_invoices function, everything besides that is from another scope.

The second test checks the case that at least one invoice was found by the task and here I found the problem. Before calling check_pending_invoices, I need to create an invoice with two specific properties: the paid flag set to false and the updated_at field with the date from at least two days ago. The first thing that comes to your mind is something like this:

After creating the instance, if you check the value of updated_at field, it won't be equal to two_days_ago. Can you see why this way doesn't work? Inside Model.objects.create(), Django will call the save() method of the model and inside of it, will look every DateTimeField with auto_now flag. If it finds any, the current datetime will be set for the field. No matter what value you set, save() will overwrite it.

In this part of Django's code, you can see that is used to get current datetime when the field is a DateTimeField. So the magic of creating a model instance in the past, is nothing more than mocking to return whatever datetime you want:

Done! We created a model instance "in the past" and now our task is 100% covered by tests. Don't forget that in the context you're using mock, hopefully only with tests, the mocked function will not work properly. If you want to learn more about it, I recommend the text Python Mocking 101: Fake It Before You Make It and, of course, the Python documentation.

Update 27/03/2017

As mentioned by @luiscastillocr on Twitter, instead of using Mock, we could use the model manager update() method. It will ignore fields with auto_now flag and, in our case, change the updated_at field. A possible side effect of this approach is that we're doing a new query on the database.