How to create dynamically generated value in PeeWee model?

In ORMs I've used so far, there was an option to add a value (field?) to model, that's not stored in database, but it's constructed using other values instead.

Here is quick example of how it could look like:

class MediaFile(BaseModel):
    ID = IntegerField()
    FileName = str(ID) + ".jpg"

So there is ID that's stored in database, and FileName that is constructed from ID. Above example of course doesn't work in PeeWee, because ID is IntegerField, not int I need.

How can I do such thing in PeeWee? How is it called?

Solution

Well, if you just want to access a property that is not stored in the database, remember that peewee models are just Python classes:

class MediaFile(BaseModel):
    ID = AutoField()  # You should probably use this for auto-inc IDs.

    @property
    def FileName(self):
        return str(self.ID) + '.jpg'

For more complex operations that you wish to be able to use both as SQL constructs and as computed instance values, see hybrid properties. An example of using hybrid properties:

class Interval(Model):
    start = IntegerField()
    end = IntegerField()

    @hybrid_property
    def length(self):
        return self.end - self.start

    @hybrid_method
    def contains(self, point):
        return (self.start <= point) & (point < self.end)

With this hybrid property and hybrid method, we can use the properties either in SQL queries or on model instances. So we end up being able to do things like:

query = Interval.select().where(Interval.contains(2))

Would generate the following SQL:

SELECT "id", "start", "end"
FROM "interval" AS t1
WHERE (("start" <= 2) AND (2 < "end"))

But we could also do:

obj = Interval(start=1, end=10)
obj.contains(2)  # returns True