django update_or_create获取“重复的键值违反唯一约束". [英] django update_or_create gets "duplicate key value violates unique constraint "

问题描述

也许我误解了Django的update_or_create Model方法的目的.

Maybe I misunderstand the purpose of Django's update_or_create Model method.

这是我的模特:

from django.db import models
import datetime
from vc.models import Cluster

class Vmt(models.Model):
    added = models.DateField(default=datetime.date.today, blank=True, null=True)
    creation_time = models.TextField(blank=True, null=True)
    current_pm_active = models.TextField(blank=True, null=True)     
    current_pm_total = models.TextField(blank=True, null=True)
    ... more simple fields ...
    cluster = models.ForeignKey(Cluster, null=True)


    class Meta:
        unique_together = (("cluster", "added"),)

这是我的考试:

from django.test import TestCase
from .models import *
from vc.models import Cluster
from django.db import transaction


# Create your tests here.
class VmtModelTests(TestCase):
    def test_insert_into_VmtModel(self):
        count = Vmt.objects.count()
        self.assertEqual(count, 0)

        # create a Cluster
        c = Cluster.objects.create(name='test-cluster')
        Vmt.objects.create(
            cluster=c,
            creation_time='test creaetion time',
            current_pm_active=5,
            current_pm_total=5,
            ... more simple fields ...
        )
        count = Vmt.objects.count()
        self.assertEqual(count, 1)
        self.assertEqual('5', c.vmt_set.all()[0].current_pm_active)

        # let's test that we cannot add that same record again
        try:
            with transaction.atomic():

                Vmt.objects.create(
                    cluster=c,
                    creation_time='test creaetion time',
                    current_pm_active=5,
                    current_pm_total=5,
                    ... more simple fields ...
                )
                self.fail(msg="Should violated integrity constraint!")
        except Exception as ex:
            template = "An exception of type {0} occurred. Arguments:\n{1!r}"
            message = template.format(type(ex).__name__, ex.args)
            self.assertEqual("An exception of type IntegrityError occurred.", message[:45])

        Vmt.objects.update_or_create(
            cluster=c,
            creation_time='test creaetion time',
            # notice we are updating current_pm_active to 6
            current_pm_active=6,
            current_pm_total=5,
            ... more simple fields ...
        )
        count = Vmt.objects.count()
        self.assertEqual(count, 1)

在上一次update_or_create调用中，出现此错误:

On the last update_or_create call I get this error:

IntegrityError: duplicate key value violates unique constraint "vmt_vmt_cluster_id_added_c2052322_uniq"
DETAIL:  Key (cluster_id, added)=(1, 2018-06-18) already exists.

为什么没有更新模型? Django为什么要尝试创建违反唯一约束的新记录?

Why didn't wasn't the model updated? Why did Django try to create a new record that violated the unique constraint?

推荐答案

The update_or_create(defaults=None, **kwargs) has basically two parts:

1. **kwargs，该标准指定"过滤器"标准，以确定是否已存在该对象；和
2. defaults是一个字典，其中包含映射到我们创建新行(如果过滤找不到行的情况下)时应使用的值的字段，或者哪些值应该进行更新(以防我们发现这样的行).

1. the **kwargs which specify the "filter" criteria to determine if such object is already present; and
2. the defaults which is a dictionary that contains the fields mapped to values that should be used when we create a new row (in case the filtering fails to find a row), or which values should be updated (in case we find such row).

这里的问题是您对过滤器设置了太严格的限制:添加了多个过滤器，结果数据库找不到该行.那么会发生什么呢?然后，数据库的目标是使用这些过滤器值创建行(并且由于缺少defaults，因此不会添加任何额外的值).但是事实证明，我们创建了一行，并且cluster和added的组合已经存在.因此，数据库拒绝添加该行.

The problem here is that you make your filters too restrictive: you add several filters, and as a result the database does not find such row. So what happens? The database then aims to create the row with these filter values (and since defaults is missing, no extra values are added). But then it turns out that we create a row, and that the combination of the cluster and added already exists. Hence the database refuses to add this row.

所以这行:

Model.objects.update_or_create(field1=val1,
                               field2=val2,
                               defaults={
                                   'field3': val3,
                                   'field4': val4
                               })

在语义上近似等于:

try:
    item = Model.objects.get(field1=val1, field2=val2)
except Model.DoesNotExist:
    Model.objects.create(field1=val1, field2=val2, field3=val3, field4=val4)
else:
    item = Model.objects.filter(
        field1=val1,
        field2=val2,
    ).update(
        field3 = val3
        field4 = val4
    )

(但原始调用通常在单个查询中完成).

(but the original call is typically done in a single query).

您可能应该这样写:

Vmt.objects.update_or_create(
    cluster=c,
    creation_time='test creaetion time',
    defaults = {        
        'current_pm_active': 6,
        'current_pm_total': 5,
    }
)

(或类似的东西)

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