Figure out child type with Django MTI or specify type as field?

ghz 9hours ago ⋅ 4 views

I'm setting up a data model in django using multiple-table inheritance (MTI) like this:

class Metric(models.Model):
    account = models.ForeignKey(Account)
    date = models.DateField()
    value = models.FloatField()
    calculation_in_progress = models.BooleanField()
    type = models.CharField( max_length=20, choices= METRIC_TYPES ) # Appropriate?

    def calculate(self):
        # default calculation...

class WebMetric(Metric):
    url = models.URLField()

    def calculate(self):
        # web-specific calculation...

class TextMetric(Metric):
    text = models.TextField()

    def calculate(self):
        # text-specific calculation...

My instinct is to put a 'type' field in the base class as shown here, so I can tell which sub-class any Metric object belongs to. It would be a bit of a hassle to keep this up to date all the time, but possible. But do I need to do this? Is there some way that django handles this automatically?

When I call Metric.objects.all() every objects returned is an instance of Metric never the subclasses. So if I call .calculate() I never get the sub-class's behavior.

I could write a function on the base class that tests to see if I can cast it to any of the sub-types like:

def determine_subtype(self):
    try:
        self.webmetric
        return WebMetric
    except WebMetric.DoesNotExist:
        pass
    # Repeat for every sub-class

but this seems like a bunch of repetitious code. And it's also not something that can be included in a SELECT filter -- only works in python-space.

What's the best way to handle this?

Answer

In Django, Multiple Table Inheritance (MTI) creates a base table for the parent model and separate tables for each of the child models. Each child model has a foreign key reference to the parent model's table, and the child-specific fields are stored in their respective tables.

Handling Subclass Behavior

You're correct that when querying the parent model (Metric.objects.all()), Django returns instances of the parent model (Metric), not the subclasses (WebMetric or TextMetric). However, Django does handle this for you, and you don’t necessarily need to manually add a type field.

Here’s what’s happening and what you can do to leverage Django’s built-in behavior:

  1. Automatic Subclass Recognition: Django automatically handles retrieving subclass instances when you query the parent model. You do not need a type field to store the subclass; Django already keeps track of which subclass the object belongs to via its internal mechanism. This means that when you query for Metric.objects.all(), Django will return instances of the appropriate subclass (WebMetric, TextMetric, etc.) if they exist.

  2. Using select_subclasses(): If you want to filter or query for specific subclasses of a model, you can use select_subclasses(), which will allow you to include subclass-specific fields in the query.

Here’s how you can modify your code:

Corrected Code

class Metric(models.Model):
    account = models.ForeignKey(Account, on_delete=models.CASCADE)
    date = models.DateField()
    value = models.FloatField()
    calculation_in_progress = models.BooleanField()
    type = models.CharField(max_length=20, choices=METRIC_TYPES)

    def calculate(self):
        # default calculation
        print("Base calculation")

class WebMetric(Metric):
    url = models.URLField()

    def calculate(self):
        # web-specific calculation
        print("Web calculation")

class TextMetric(Metric):
    text = models.TextField()

    def calculate(self):
        # text-specific calculation
        print("Text calculation")

Key Points:

  • Subclasses are automatically handled: When you call Metric.objects.all(), Django will return instances of Metric, WebMetric, or TextMetric depending on what type of row is in the database.

  • Calling calculate(): If you call calculate() on an instance, Django will automatically invoke the calculate() method of the specific subclass if the instance is a WebMetric or TextMetric.

    For example:

    metric = Metric.objects.get(id=1)
metric.calculate()  # Will call WebMetric.calculate() or TextMetric.calculate() based on the type of instance.

    You don’t need to manually check for the type of the object; Django will handle the method resolution correctly.

Efficient Queries with select_subclasses()

If you want to efficiently retrieve specific fields from child models, you can use select_subclasses() to prefetch subclass data. For example:

metrics = Metric.objects.select_subclasses(WebMetric, TextMetric).all()

This query ensures that both parent and subclass data are retrieved in one go, and it will return the appropriate subclass instance based on the actual type in the database.

Subclassing and calculate() Method

When you define a method in the parent (Metric) like calculate(), it can be overridden in each subclass (WebMetric, TextMetric). When you call metric.calculate(), the correct method for the subclass will be executed. You don’t need to manually check the type of object or add a type field to identify it.

If you want to differentiate based on the type in your base class, you can use isinstance() or model._meta.model_name (which gives you the model name) for more advanced cases, but in general, Django handles this automatically.

Example with isinstance():

If you want to check explicitly which subclass an object belongs to, you can use isinstance():

metric = Metric.objects.get(id=1)

if isinstance(metric, WebMetric):
    metric.calculate()  # Calls WebMetric's calculate()
elif isinstance(metric, TextMetric):
    metric.calculate()  # Calls TextMetric's calculate()
else:
    metric.calculate()  # Calls Metric's default calculate()

No Need for type Field

The type field that you initially thought of adding is unnecessary. Django's MTI handles subclassing automatically, and the correct subclass behavior is invoked when you call methods like calculate(). The type field might be redundant unless you need to store the type explicitly for other purposes (e.g., filtering or reporting).

Conclusion:

  • Django handles the inheritance and subclass resolution automatically.
  • You don't need to store a type field, as Django keeps track of the subclass and will call the correct calculate() method.
  • You can use select_subclasses() for efficient queries if you need to include subclass-specific fields in your queries.
#py