Django 保存时，如何检查字段是否已更改？

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

这是

远程图像第一次改变，效果非常好

如果有人修改了别名上的
远程\u图像
，如何获取新图像？第二，有没有更好的方法来缓存远程图像？
虽然这实际上并不能回答您的问题，但我会用另一种方法来解决这个问题

成功保存本地副本后，只需清除
remote\u image
字段即可。然后在保存方法中，只要
remote\u image
不为空，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来处理缓存标志，而不是
remote\u image
字段本身。
作为SmileyChris答案的扩展，您可以在模型中添加datetime字段以供上次更新，在检查更改之前，为字段的最大年龄设置某种限制，现在是直接回答：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)

使用表单时，同样的情况也适用。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 = []

基本上，您希望覆盖
models.Model
的
\uuuu init\uuu
方法，以便保留原始值的副本。这使得您不必再进行另一次数据库查找（这总是一件好事）

最好的方法是使用
pre_save
信号。早在09年这个问题被问到和回答时，这可能不是一个选项，但今天看到这一点的人应该这样做：

@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

我使用以下mixin：

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
请注意，此解决方案仅在当前请求的上下文中运行良好。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，您肯定需要一种不同的方法。
在我的解决方案覆盖目标字段类的
pre_save（）
方法之前，我就遇到过这种情况，只有在字段发生更改时才会调用它

与FileField一起使用很有用
示例：


缺点：

如果您想执行任何（post_save）操作，如在某些作业中使用创建的对象（如果某些字段已更改），则此操作将不起作用。
我已将@livskiy的混入扩展如下：

class ModelDiffMixin（models.Model）：
"""
跟踪模型字段值并提供一些有用api的模型混合
了解哪些字段已更改。
"""
_dict=DictField（可编辑=False）
定义初始化（self，*args，**kwargs）：
super（ModelDiffMixin，self）。\uuuuuu初始值（*args，**kwargs）
self.\u首字母=self.\u dict
@财产
def差异（自身）：
d1=自身首字母
d2=自身
d1中k，v的差异=[（k，（v，d2[k]）。如果v！=d2[k]]，则为items（）
返回指令（差异）
@财产
def已更改（自身）：
返回布尔值（自身差异）
@财产
def更改_字段（自身）：
返回自差异键（）
def get_field_diff（自身、字段名称）：
"""
如果字段已更改，则返回一个diff for字段，否则不返回。
"""
返回self.diff.get（字段名称，无）
def保存（自身、*args、**kwargs）：
"""
保存模型并设置初始状态。
"""
对象dict=模型dict（自我，
fields=[field.name用于self.\u meta.fields]中的字段）
对于对象目录中的字段：
#用于文件字段
如果issubclass（对象目录[字段]。\uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuu
尝试：
object\u dict[field]=object\u dict[field]。路径
除：
object\u dict[field]=object\u dict[field]。名称
#TODO:添加其他不可序列化的字段类型
self.\u dict=对象\u dict
super（ModelDiffMixin，self）.save（*args，**kwargs）
类元：
抽象=真

这个字段是：

类字段（models.TextField）：
__元类=模型.SubfieldBase
description=“存储python dict”
定义初始化（self，*args，**kwargs）：
超级（DictField，self）。\uuuuuuuuu初始化（*args，**kwargs）
定义到python（自身，值）：
如果不是值：
值={}
如果isinstance（值，dict）：
返回值
返回json.loads（值）
def get_prep_值（自身，值）：
如果值为“无”：
返回值
返回json.dumps（值）
def值_至_字符串（自身，obj）：
值=自身。从对象（对象）获取值
返回self.get_db_prep_值（值）

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
为所有字段改进@josh answer：

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)

为了澄清这一点，getattr可以使用字符串获取像
person.name
这样的字段（即
getattr（person，“name”）
您可以使用它来执行此操作，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

另一个迟来的答案，但如果你只是想看看是否有一个新文件已上传到文件字段，试试这个：（改编自克里斯托弗·亚当斯在扎克评论中对链接的评论）

更新链接：

来自@ivanlivski的混音很棒

我把它扩展到


确保它适用于十进制字段
公开属性以简化使用

更新的代码可在以下位置获得：

为了帮助不熟悉Python或Django的人，我将给出一个更完整的示例。
这种特殊用法是从数据提供者获取文件，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

类站（ModelDiffMixin.ModelDiffMixin，models.Model）：
站点名称=models.CharField（最大长度=200）
附近城市=models.CharField（最大长度）=
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

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

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

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()

@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!"

@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!

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

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

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)

@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

@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))
    )

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)

  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()

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)

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)

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

@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