保存时，如何检查字段是否已更改？

Question

在我的模型中，我有：

class Alias(MyBaseModel):
    remote_image = models.URLField(max_length=500, null=True, help_text="A URL that is downloaded and cached for the image. Only
 used when the alias is made")
    image = models.ImageField(upload_to='alias', default='alias-default.png', help_text="An image representing the alias")


    def save(self, *args, **kw):
        if (not self.image or self.image.name == 'alias-default.png') and self.remote_image :
            try :
                data = utils.fetch(self.remote_image)
                image = StringIO.StringIO(data)
                image = Image.open(image)
                buf = StringIO.StringIO()
                image.save(buf, format='PNG')
                self.image.save(hashlib.md5(self.string_id).hexdigest() + ".png", ContentFile(buf.getvalue()))
            except IOError :
                pass

remote_image 第一次更改时效果很好。

当有人修改了别名上的 remote_image 时，我如何获取新图像？其次，有没有更好的方法来缓存远程图像？

Answer 1

本质上，您希望覆盖models.Model 的__init__ 方法，以便保留原始值的副本。这样您就不必再进行一次数据库查找（这总是一件好事）。

    class Person(models.Model):
        name = models.CharField()

        __original_name = None

        def __init__(self, *args, **kwargs):
            super().__init__(*args, **kwargs)
            self.__original_name = self.name

        def save(self, force_insert=False, force_update=False, *args, **kwargs):
            if self.name != self.__original_name:
                # name changed - do something here

            super().save(force_insert, force_update, *args, **kwargs)
            self.__original_name = self.name

Answer 2

我使用以下混合：

from django.forms.models import model_to_dict


class ModelDiffMixin(object):
    """
    A model mixin that tracks model fields' values and provide some useful api
    to know what fields have been changed.
    """

    def __init__(self, *args, **kwargs):
        super(ModelDiffMixin, self).__init__(*args, **kwargs)
        self.__initial = self._dict

    @property
    def diff(self):
        d1 = self.__initial
        d2 = self._dict
        diffs = [(k, (v, d2[k])) for k, v in d1.items() if v != d2[k]]
        return dict(diffs)

    @property
    def has_changed(self):
        return bool(self.diff)

    @property
    def changed_fields(self):
        return self.diff.keys()

    def get_field_diff(self, field_name):
        """
        Returns a diff for field if it's changed and None otherwise.
        """
        return self.diff.get(field_name, None)

    def save(self, *args, **kwargs):
        """
        Saves model and set initial state.
        """
        super(ModelDiffMixin, self).save(*args, **kwargs)
        self.__initial = self._dict

    @property
    def _dict(self):
        return model_to_dict(self, fields=[field.name for field in
                             self._meta.fields])

用法：

>>> p = Place()
>>> p.has_changed
False
>>> p.changed_fields
[]
>>> p.rank = 42
>>> p.has_changed
True
>>> p.changed_fields
['rank']
>>> p.diff
{'rank': (0, 42)}
>>> p.categories = [1, 3, 5]
>>> p.diff
{'categories': (None, [1, 3, 5]), 'rank': (0, 42)}
>>> p.get_field_diff('categories')
(None, [1, 3, 5])
>>> p.get_field_diff('rank')
(0, 42)
>>>

---

注意

请注意，此解决方案仅适用于当前请求的上下文。因此它主要适用于简单的情况。在多个请求可以同时操作同一个模型实例的并发环境中，您肯定需要不同的方法。

Answer 3

最好的方法是使用pre_save 信号。在 09 年提出和回答这个问题时，可能不是一个选择，但今天看到这个问题的任何人都应该这样做：

@receiver(pre_save, sender=MyModel)
def do_something_if_changed(sender, instance, **kwargs):
    try:
        obj = sender.objects.get(pk=instance.pk)
    except sender.DoesNotExist:
        pass # Object is new, so field hasn't technically changed, but you may want to do something else here.
    else:
        if not obj.some_field == instance.some_field: # Field has changed
            # do something

Answer 4

现在直接回答：检查字段值是否已更改的一种方法是在保存实例之前从数据库中获取原始数据。考虑这个例子：

class MyModel(models.Model):
    f1 = models.CharField(max_length=1)

    def save(self, *args, **kw):
        if self.pk is not None:
            orig = MyModel.objects.get(pk=self.pk)
            if orig.f1 != self.f1:
                print 'f1 changed'
        super(MyModel, self).save(*args, **kw)

在处理表单时也是如此。您可以在 ModelForm 的 clean 或 save 方法中检测到它：

class MyModelForm(forms.ModelForm):

    def clean(self):
        cleaned_data = super(ProjectForm, self).clean()
        #if self.has_changed():  # new instance or existing updated (form has data to save)
        if self.instance.pk is not None:  # new instance only
            if self.instance.f1 != cleaned_data['f1']:
                print 'f1 changed'
        return cleaned_data

    class Meta:
        model = MyModel
        exclude = []

Answer 5

自 Django 1.8 发布以来，您可以使用 from_db 类方法来缓存 remote_image 的旧值。然后在 save 方法中，您可以比较字段的新旧值以检查该值是否已更改。

@classmethod
def from_db(cls, db, field_names, values):
    new = super(Alias, cls).from_db(db, field_names, values)
    # cache value went from the base
    new._loaded_remote_image = values[field_names.index('remote_image')]
    return new

def save(self, force_insert=False, force_update=False, using=None,
         update_fields=None):
    if (self._state.adding and self.remote_image) or \
        (not self._state.adding and self._loaded_remote_image != self.remote_image):
        # If it is first save and there is no cached remote_image but there is new one, 
        # or the value of remote_image has changed - do your stuff!

Answer 6

请注意，django-model-utils 中提供了字段更改跟踪。

https://django-model-utils.readthedocs.org/en/latest/index.html

Answer 7

如果您使用的是表单，则可以使用表单的changed_data (docs)：

class AliasForm(ModelForm):

    def save(self, commit=True):
        if 'remote_image' in self.changed_data:
            # do things
            remote_image = self.cleaned_data['remote_image']
            do_things(remote_image)
        super(AliasForm, self).save(commit)

    class Meta:
        model = Alias

Answer 8

我参加聚会有点晚了，但我也找到了这个解决方案： Django Dirty Fields

Answer 9

另一个较晚的答案，但如果您只是想查看是否有新文件已上传到文件字段，请尝试以下操作：（改编自 Christopher Adams 对链接 http://zmsmith.com/2010/05/django-check-if-a-field-has-changed/ 在 zach 的评论中的评论）

更新链接：https://web.archive.org/web/20130101010327/http://zmsmith.com:80/2010/05/django-check-if-a-field-has-changed/

def save(self, *args, **kw):
    from django.core.files.uploadedfile import UploadedFile
    if hasattr(self.image, 'file') and isinstance(self.image.file, UploadedFile) :
        # Handle FileFields as special cases, because the uploaded filename could be
        # the same as the filename that's already there even though there may
        # be different file contents.

        # if a file was just uploaded, the storage model with be UploadedFile
        # Do new file stuff here
        pass

Answer 10

正如 Serge 提到的，从 Django 1.8 开始，有 from_db 方法。事实上，Django 文档以这个特定的用例为例：

https://docs.djangoproject.com/en/dev/ref/models/instances/#customizing-model-loading

下面是一个示例，显示如何记录从数据库加载的字段的初始值

Answer 11

这在 Django 1.8 中适用于我

def clean(self):
    if self.cleaned_data['name'] != self.initial['name']:
        # Do something

Answer 12

有一个属性 __dict__ ，它的所有字段都作为键，值作为字段值。所以我们可以比较其中两个

只需将模型的保存功能改成下面的功能

def save(self, force_insert=False, force_update=False, using=None, update_fields=None):
    if self.pk is not None:
        initial = A.objects.get(pk=self.pk)
        initial_json, final_json = initial.__dict__.copy(), self.__dict__.copy()
        initial_json.pop('_state'), final_json.pop('_state')
        only_changed_fields = {k: {'final_value': final_json[k], 'initial_value': initial_json[k]} for k in initial_json if final_json[k] != initial_json[k]}
        print(only_changed_fields)
    super(A, self).save(force_insert=False, force_update=False, using=None, update_fields=None)

示例用法：

class A(models.Model):
    name = models.CharField(max_length=200, null=True, blank=True)
    senior = models.CharField(choices=choices, max_length=3)
    timestamp = models.DateTimeField(null=True, blank=True)

    def save(self, force_insert=False, force_update=False, using=None, update_fields=None):
        if self.pk is not None:
            initial = A.objects.get(pk=self.pk)
            initial_json, final_json = initial.__dict__.copy(), self.__dict__.copy()
            initial_json.pop('_state'), final_json.pop('_state')
            only_changed_fields = {k: {'final_value': final_json[k], 'initial_value': initial_json[k]} for k in initial_json if final_json[k] != initial_json[k]}
            print(only_changed_fields)
        super(A, self).save(force_insert=False, force_update=False, using=None, update_fields=None)

仅输出那些已更改的字段

{'name': {'initial_value': '1234515', 'final_value': 'nim'}, 'senior': {'initial_value': 'no', 'final_value': 'yes'}}

Answer 13

您可以使用django-model-changes 来执行此操作，而无需额外的数据库查找：

from django.dispatch import receiver
from django_model_changes import ChangesMixin

class Alias(ChangesMixin, MyBaseModel):
   # your model

@receiver(pre_save, sender=Alias)
def do_something_if_changed(sender, instance, **kwargs):
    if 'remote_image' in instance.changes():
        # do something

Answer 14

游戏已经很晚了，但这是Chris Pratt's answer 的一个版本，它通过使用transaction 块和select_for_update() 来防止竞争条件，同时牺牲性能。

@receiver(pre_save, sender=MyModel)
@transaction.atomic
def do_something_if_changed(sender, instance, **kwargs):
    try:
        obj = sender.objects.select_for_update().get(pk=instance.pk)
    except sender.DoesNotExist:
        pass # Object is new, so field hasn't technically changed, but you may want to do something else here.
    else:
        if not obj.some_field == instance.some_field: # Field has changed
            # do something

Answer 15

最佳解决方案可能是在保存模型实例之前不包括额外的数据库读取操作，也不包括任何进一步的 django-library。这就是为什么 laffuste 的解决方案更可取的原因。在管理站点的上下文中，可以简单地覆盖save_model-方法，并在那里调用表单的has_changed 方法，就像上面 Sion 的回答一样。你会得到这样的结果，借鉴 Sion 的示例设置，但使用 changed_data 进行所有可能的更改：

class ModelAdmin(admin.ModelAdmin):
   fields=['name','mode']
   def save_model(self, request, obj, form, change):
     form.changed_data #output could be ['name']
     #do somethin the changed name value...
     #call the super method
     super(self,ModelAdmin).save_model(request, obj, form, change)

• 覆盖save_model:

https://docs.djangoproject.com/en/1.10/ref/contrib/admin/#django.contrib.admin.ModelAdmin.save_model

• 内置changed_data-字段方法：

https://docs.djangoproject.com/en/1.10/ref/forms/api/#django.forms.Form.changed_data

Answer 16

虽然这实际上并不能回答您的问题，但我会以不同的方式解决这个问题。

成功保存本地副本后，只需清除remote_image 字段即可。然后在您的保存方法中，只要remote_image 不为空，您就可以随时更新图像。

如果您想保留对 url 的引用，可以使用不可编辑的布尔字段而不是 remote_image 字段本身来处理缓存标志。

Answer 17

在我的解决方案是覆盖目标字段类的pre_save() 方法之前，我遇到过这种情况，只有在字段已更改时才会调用它
对 FileField 有用 示例：

class PDFField(FileField):
    def pre_save(self, model_instance, add):
        # do some operations on your file 
        # if and only if you have changed the filefield

缺点：
如果您想做任何（post_save）操作，例如在某些作业中使用创建的对象（如果某些字段已更改），则没有用

Answer 18

我对@livskiy 的mixin 进行了如下扩展：

class ModelDiffMixin(models.Model):
    """
    A model mixin that tracks model fields' values and provide some useful api
    to know what fields have been changed.
    """
    _dict = DictField(editable=False)
    def __init__(self, *args, **kwargs):
        super(ModelDiffMixin, self).__init__(*args, **kwargs)
        self._initial = self._dict

    @property
    def diff(self):
        d1 = self._initial
        d2 = self._dict
        diffs = [(k, (v, d2[k])) for k, v in d1.items() if v != d2[k]]
        return dict(diffs)

    @property
    def has_changed(self):
        return bool(self.diff)

    @property
    def changed_fields(self):
        return self.diff.keys()

    def get_field_diff(self, field_name):
        """
        Returns a diff for field if it's changed and None otherwise.
        """
        return self.diff.get(field_name, None)

    def save(self, *args, **kwargs):
        """
        Saves model and set initial state.
        """
        object_dict = model_to_dict(self,
               fields=[field.name for field in self._meta.fields])
        for field in object_dict:
            # for FileFields
            if issubclass(object_dict[field].__class__, FieldFile):
                try:
                    object_dict[field] = object_dict[field].path
                except :
                    object_dict[field] = object_dict[field].name

            # TODO: add other non-serializable field types
        self._dict = object_dict
        super(ModelDiffMixin, self).save(*args, **kwargs)

    class Meta:
        abstract = True

DictField 是：

class DictField(models.TextField):
    __metaclass__ = models.SubfieldBase
    description = "Stores a python dict"

    def __init__(self, *args, **kwargs):
        super(DictField, self).__init__(*args, **kwargs)

    def to_python(self, value):
        if not value:
            value = {}

        if isinstance(value, dict):
            return value

        return json.loads(value)

    def get_prep_value(self, value):
        if value is None:
            return value
        return json.dumps(value)

    def value_to_string(self, obj):
        value = self._get_val_from_obj(obj)
        return self.get_db_prep_value(value)

可以通过在模型中扩展它来使用它 同步/迁移时将添加一个 _dict 字段，该字段将存储对象的状态

Answer 19

改进所有领域的@josh 答案：

class Person(models.Model):
  name = models.CharField()

def __init__(self, *args, **kwargs):
    super(Person, self).__init__(*args, **kwargs)
    self._original_fields = dict([(field.attname, getattr(self, field.attname))
        for field in self._meta.local_fields if not isinstance(field, models.ForeignKey)])

def save(self, *args, **kwargs):
  if self.id:
    for field in self._meta.local_fields:
      if not isinstance(field, models.ForeignKey) and\
        self._original_fields[field.name] != getattr(self, field.name):
        # Do Something    
  super(Person, self).save(*args, **kwargs)

澄清一下，getattr 用于获取带有字符串的person.name 之类的字段（即getattr(person, "name")

Answer 20

使用 David Cramer 的解决方案怎么样：

http://cramer.io/2010/12/06/tracking-changes-to-fields-in-django/

我已经成功地像这样使用它：

@track_data('name')
class Mode(models.Model):
    name = models.CharField(max_length=5)
    mode = models.CharField(max_length=5)

    def save(self, *args, **kwargs):
        if self.has_changed('name'):
            print 'name changed'

    # OR #

    @classmethod
    def post_save(cls, sender, instance, created, **kwargs):
        if instance.has_changed('name'):
            print "Hooray!"

Answer 21

对@ivanperelivskiy 答案的修改：

@property
def _dict(self):
    ret = {}
    for field in self._meta.get_fields():
        if isinstance(field, ForeignObjectRel):
            # foreign objects might not have corresponding objects in the database.
            if hasattr(self, field.get_accessor_name()):
                ret[field.get_accessor_name()] = getattr(self, field.get_accessor_name())
            else:
                ret[field.get_accessor_name()] = None
        else:
            ret[field.attname] = getattr(self, field.attname)
    return ret

这使用了 django 1.10 的公共方法get_fields。这使代码更具前瞻性，但更重要的是还包括外键和可编辑=假的字段。

供参考，这里是.fields的实现

@cached_property
def fields(self):
    """
    Returns a list of all forward fields on the model and its parents,
    excluding ManyToManyFields.

    Private API intended only to be used by Django itself; get_fields()
    combined with filtering of field properties is the public API for
    obtaining this field list.
    """
    # For legacy reasons, the fields property should only contain forward
    # fields that are not private or with a m2m cardinality. Therefore we
    # pass these three filters as filters to the generator.
    # The third lambda is a longwinded way of checking f.related_model - we don't
    # use that property directly because related_model is a cached property,
    # and all the models may not have been loaded yet; we don't want to cache
    # the string reference to the related_model.
    def is_not_an_m2m_field(f):
        return not (f.is_relation and f.many_to_many)

    def is_not_a_generic_relation(f):
        return not (f.is_relation and f.one_to_many)

    def is_not_a_generic_foreign_key(f):
        return not (
            f.is_relation and f.many_to_one and not (hasattr(f.remote_field, 'model') and f.remote_field.model)
        )

    return make_immutable_fields_list(
        "fields",
        (f for f in self._get_fields(reverse=False)
         if is_not_an_m2m_field(f) and is_not_a_generic_relation(f) and is_not_a_generic_foreign_key(f))
    )

Answer 22

这是另一种方法。

class Parameter(models.Model):

    def __init__(self, *args, **kwargs):
        super(Parameter, self).__init__(*args, **kwargs)
        self.__original_value = self.value

    def clean(self,*args,**kwargs):
        if self.__original_value == self.value:
            print("igual")
        else:
            print("distinto")

    def save(self,*args,**kwargs):
        self.full_clean()
        return super(Parameter, self).save(*args, **kwargs)
        self.__original_value = self.value

    key = models.CharField(max_length=24, db_index=True, unique=True)
    value = models.CharField(max_length=128)

根据文档：validating objects

“full_clean() 执行的第二步是调用 Model.clean()。应该重写此方法以对您的模型执行自定义验证。 此方法应用于提供自定义模型验证，并根据需要修改模型上的属性。例如，您可以使用它来自动为字段提供值，或者进行需要访问多个字段的验证："

Answer 23

我对@iperelivskiy 的解决方案的看法：在大规模上，为每个__init__ 创建_initial dict 是昂贵的，而且大多数时候是不必要的。我稍微更改了 mixin，以便它仅在您明确告诉它这样做时才记录更改（通过调用 instance.track_changes）：

from typing import KeysView, Optional
from django.forms import model_to_dict

class TrackChangesMixin:
    _snapshot: Optional[dict] = None

    def track_changes(self):
        self._snapshot = self.as_dict

    @property
    def diff(self) -> dict:
        if self._snapshot is None:
            raise ValueError("track_changes wasn't called, can't determine diff.")
        d1 = self._snapshot
        d2 = self.as_dict
        diffs = [(k, (v, d2[k])) for k, v in d1.items() if str(v) != str(d2[k])]
        return dict(diffs)

    @property
    def has_changed(self) -> bool:
        return bool(self.diff)

    @property
    def changed_fields(self) -> KeysView:
        return self.diff.keys()

    @property
    def as_dict(self) -> dict:
        return model_to_dict(self, fields=[field.name for field in self._meta.fields])

Answer 24

作为 SmileyChris 回答的扩展，您可以为 last_updated 向模型添加一个日期时间字段，并为在检查更改之前允许它达到的最大年龄设置某种限制

Answer 25

@ivanlivski 的 mixin 很棒。

我已经把它扩展到

• 确保它适用于小数字段。
• 公开属性以简化使用

更新的代码可在此处获得： https://github.com/sknutsonsf/python-contrib/blob/master/src/django/utils/ModelDiffMixin.py

为了帮助刚接触 Python 或 Django 的人，我将给出一个更完整的示例。 这种特殊用途是从数据提供者处获取文件并确保数据库中的记录反映该文件。

我的模型对象：

class Station(ModelDiffMixin.ModelDiffMixin, models.Model):
    station_name = models.CharField(max_length=200)
    nearby_city = models.CharField(max_length=200)

    precipitation = models.DecimalField(max_digits=5, decimal_places=2)
    # <list of many other fields>

   def is_float_changed (self,v1, v2):
        ''' Compare two floating values to just two digit precision
        Override Default precision is 5 digits
        '''
        return abs (round (v1 - v2, 2)) > 0.01

加载文件的类有这些方法：

class UpdateWeather (object)
    # other methods omitted

    def update_stations (self, filename):
        # read all existing data 
        all_stations = models.Station.objects.all()
        self._existing_stations = {}

        # insert into a collection for referencing while we check if data exists
        for stn in all_stations.iterator():
            self._existing_stations[stn.id] = stn

        # read the file. result is array of objects in known column order
        data = read_tabbed_file(filename)

        # iterate rows from file and insert or update where needed
        for rownum in range(sh.nrows):
            self._update_row(sh.row(rownum));

        # now anything remaining in the collection is no longer active
        # since it was not found in the newest file
        # for now, delete that record
        # there should never be any of these if the file was created properly
        for stn in self._existing_stations.values():
            stn.delete()
            self._num_deleted = self._num_deleted+1


    def _update_row (self, rowdata):
        stnid = int(rowdata[0].value) 
        name = rowdata[1].value.strip()

        # skip the blank names where data source has ids with no data today
        if len(name) < 1:
            return

        # fetch rest of fields and do sanity test
        nearby_city = rowdata[2].value.strip()
        precip = rowdata[3].value

        if stnid in self._existing_stations:
            stn = self._existing_stations[stnid]
            del self._existing_stations[stnid]
            is_update = True;
        else:
            stn = models.Station()
            is_update = False;

        # object is new or old, don't care here            
        stn.id = stnid
        stn.station_name = name;
        stn.nearby_city = nearby_city
        stn.precipitation = precip

        # many other fields updated from the file 

        if is_update == True:

            # we use a model mixin to simplify detection of changes
            # at the cost of extra memory to store the objects            
            if stn.has_changed == True:
                self._num_updated = self._num_updated + 1;
                stn.save();
        else:
            self._num_created = self._num_created + 1;
            stn.save()

Answer 26

如果您对重写 save 方法不感兴趣，您可以这样做

  model_fields = [f.name for f in YourModel._meta.get_fields()]
  valid_data = {
        key: new_data[key]
        for key in model_fields
        if key in new_data.keys()
  }

  for (key, value) in valid_data.items():
        if getattr(instance, key) != value:
           print ('Data has changed')

        setattr(instance, key, value)

 instance.save()

Answer 27

我找到了这个包django-lifecycle。 它使用 django 信号来定义@hook 装饰器，非常健壮和可靠。我用过，很幸福。