Question

是否有最好的＆＃34;做可以查询的计算字段的方法吗？

例如：

from django.db import models

class Person(models.Model):
    given_name = models.CharField(max_length=30)
    family_name = models.CharField(max_length=30)

    NAME_ORDER_CONVENTION_CHOICES = (
        # "Eastern" name order: family name followed by given name
        ('E', 'Eastern'),
        # "Western" name order: given name followed by family name
        ('W', 'Western')
    )

    name_order_convention = models.CharField(
        length=1, 
        choices=NAME_ORDER_CONVENTION_CHOICES,
    )

    @property
    def full_name(self):
        """
        Return full name, calculated on the basis of given name,
        family name and name order convention.
        """
        template = "{} {}"
        if self.name_order_convention == "W":
            return template.format(self.given_name, self.family_name)
        return template.format(self.family_name, self.given_name)

这使您能够为任何full_name获取Person，但如果您想基于全名进行数据库查询，则需要编写一个隐含的如何知道查询的查询此属性是计算的。这似乎违反了DRY，因为不再有任何中心位置可以更改计算字段的计算方式 - 您必须更改full_name属性，以及任何位置查询基于full_name如何工作的隐含知识。

我能想到的主要替代方法是覆盖save()方法以在每次更新时更新这些字段。

from django.db import models

class Person(models.Model):
    given_name = models.CharField(max_length=30)
    family_name = models.CharField(max_length=30)

    NAME_ORDER_CONVENTION_CHOICES = (
        ('E', 'Eastern'),
        ('W', 'Western')
    )

    name_order_convention = models.CharField(
        length=1, 
        choices=NAME_ORDER_CONVENTION_CHOICES,
    )

    # Computed fields
    full_name = models.CharField(max_length=60)

    def _calculate_full_name(self):
        template = "{} {}"
        if self.name_order_convention == "W":
            self.full_name = template.format(self.given_name, self.family_name)
        else:
            self.full_name = template.format(self.family_name, self.given_name)

    def save(self, *args, **kwargs):
        self._calculate_full_name()
        super(Model, self).save(*args, **kwargs)

是否有共同的，最好的做法＆＃34;我可能会遗漏的可查询计算字段的解决方案？

Answer 1

我可以看到的一种方式：在数据库中使用连接的full_name创建视图，而不是用于查询的django中的托管模型（Person模型也应存在于crud操作中）：

view（postgresql）

CREATE OR REPLACE VIEW appname_viewperson AS 
    SELECT id,
        given_name,
        family_name,
        given_name || ' ' || family_name AS full_name,
    FROM appname_person

非管理型号：

class ViewPerson(models.Model):
    given_name = models.CharField(max_length=30)
    family_name = models.CharField(max_length=30)
    full_name = models.CharField(max_length=60)

    class Meta:
        managed = False

Answer 2

答案可能在于django相当新的查询注释：

https://docs.djangoproject.com/en/1.11/ref/models/expressions/#using-f-with-annotations

company = Company.objects.annotate(
    chairs_needed=F('num_employees') - F('num_chairs'))

Django：可查询的计算字段

2 个答案: