1

是否有一种“最好”的方式来处理可以查询的计算字段？

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)

这使您能够获取full_namefor any Person，但是如果您想根据全名进行数据库查询，则需要编写一个隐含知道如何计算此属性的查询。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属性以及基于隐式知识进行查询的任何位置是如何full_name工作的。

我能想到的主要替代save()方法是覆盖每次更新时更新这些字段的方法。

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)

对于我可能遗漏的可查询计算字段，是否有一个常见的“最佳实践”解决方案？

4

2 回答 2

3

我可以看到的一种方法：在数据库中创建具有连接 full_name 的视图，而不是在 django 中用于查询的托管模型（Person模型也应该存在于 crud 操作）：

查看（postgresql）

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

非托管模型：

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
于 2018-02-28T15:14:02.253 回答
1

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'))
于 2018-03-21T11:24:42.933 回答