python - Django how to call a method from a custom field given a model instance?

Question

I have the following model:

class CustomField(models.CharField):
    def foo(self):
        return 'foo'

class Test(models.Model):
    col1 = models.CharField(max_length=45)
    col2 = CustomField(max_length=45)

How can I call the foo method from CustomField, if I'm given an instance of Test?

For example:

>>> t = Test.objects.create(col1='bar', col2='blah')
>>> t.col2
'blah'
>>> t.col2.foo() # 'str' object has not attribute 'foo'
'foo'

This, of course, throws:

'str' object has not attribute 'foo'

because calling model_instance.column returns the value of that column, not an instance of column.

But why exactly? It seems Django's ORM magically transforms an instance of a field class into a value. I've spent hours digging through source code and can't seem to find where the transformation takes place.

TLDR;

Is it possible to return an instance of a field class given a model instance?

Any idea where this happens in Django's source code? I assume this takes place in django/db/models/base.py, but that file is over 1800 lines of code, so it's really hard to tell.

Update

Here is a practical example of why this would be useful:

class TempField(models.DecimalField):
    def __init__(self, initial_unit='C', **kwargs):
        self.initial_unit = initial_unit
        self.units = ['F', 'C', 'K']

    def convert(self, unit):
        if self.initial_unit == unit:
            return self.value

        if unit not in self.units:
            raise

        attr = getattr(self, f'_{initial_unit}_to_{unit}', None)
        if attr is None:
            raise

        return attr(unit)

        def _C_to_F(self, unit):
            ...

Now you can conveniently convert this field to the desired unit:

class Test(models.Model):
    temperature = TempField(...)

>>>t = Test.objects.create(temperature=100)
>>>t.temperature
100
>>>t.temperature.convert('F')
212

This is all just untested pseudo code. Also, I can think of several ways of having this functionality without the headache of using custom fields in this manner; so this question is really about understanding how Django's ORM works, and not necessarily how to solve any real world problems.

标签： pythondjangodjango-models

Answer 1

David Wheeler在计算机科学中有一句名言，“计算机科学中的所有问题都可以通过另一个层次的间接来解决（除了太多的间接层） ”。

因此，我们可以定义一个类Temperature来存储温度：

from enum import Enum
from decimal import Decimal
NINE_FIFTHS = Decimal(9)/Decimal(5)

class TemperatureUnit(Enum):
    KELVIN = (1,0, 'K')
    FAHRENHEIT = (NINE_FIFTHS, Decimal('-459.67'), '°F')
    CELSIUS = (1, Decimal('-273.15'), '°C')
    RANKINE = (NINE_FIFTHS, 0, '°R')

class Temperature:

    def __init__(self, kelvin, unit=TemperatureUnit.CELSIUS):
        self.kelvin = Decimal(kelvin)
        self.unit = unit

    @staticmethod
    def from_unit(value, unit=TemperatureUnit.CELSIUS):
        a, b, *__ = unit.value
        return Temperature((value-b)/a, unit)

    @property
    def value(self):
        a, b, *__ = self.unit.value
        return a * self.kelvin + b

    def convert(self, unit):
        return Temperature(self.kelvin, unit)

    def __str__(self):
        return '{} {}'.format(self.value, self.unit.value[2])

例如，我们可以在这里创建温度：

>>> str(Temperature(15, unit=TemperatureUnit.FAHRENHEIT))
'-432.67 °F'
>>> str(Temperature(0, unit=TemperatureUnit.FAHRENHEIT))
'-459.67 °F'
>>> str(Temperature(1, unit=TemperatureUnit.FAHRENHEIT))
'-457.87 °F'
>>> str(Temperature(0, unit=TemperatureUnit.FAHRENHEIT))
'-459.67 °F'
>>> str(Temperature(0, unit=TemperatureUnit.CELSIUS))
'-273.15 °C'

现在我们可以创建一个存储和检索Temperatures 的 Django 模型字段，例如通过将这些保存在数据库端的小数中，在开尔文中：

class TemperatureField(models.DecimalField):

    def from_db_value(self, value):
        kelvin = super().from_db_value(value)
        if kelvin is not None:
            return Temperature(kelvin)
        return None

    def to_python(self, value):
        if isinstance(value, Temperature):
            return value
        if value is None:
            return value
        kelvin = super().to_python(value)
        return Temperature(kelvin)

    def get_prep_value(self, value):
        if isinstance(value, Temperature):
            value = value.kelvin
        return super().get_prep_value(value)

以上当然是原始草图。有关详细信息，请参阅有关编写自定义模型字段的文档。您可以添加表单字段、小部件、查询数据库等。因此您可以为您的TemperatureField.