When saving how can you check if a field has changed

Question

In my model I have    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   Which works great for the first time the remote image changes    How can I fetch a new image when someone has modified the remote image on the alias  And secondly  is there a better way to cache a remote image

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Since Django 1 8 released  you can use from db classmethod to cache old value of remote image  Then in save method you can compare old and new value of field to check if the value has changed    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

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Very late to the game  but this is a version of Chris Pratt s answer that protects against race conditions while sacrificing performance  by using a transaction block and select for update     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

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as an extension of SmileyChris  answer  you can add a datetime field to the model for last updated  and set some sort of limit for the max age you ll let it get to before checking for a change

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If you do not find interest in overriding save method  you can do    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

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While this doesn t actually answer your question  I d go about this in a different way   Simply clear the remote image field after successfully saving the local copy  Then in your save method you can always update the image whenever remote image isn t empty   If you d like to keep a reference to the url  you could use an non-editable boolean field to handle the caching flag rather than remote image field itself

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Essentially  you want to override the   init   method of models Model so that you keep a copy of the original value   This makes it so that you don t have to do another DB lookup  which is always a good thing    class Person models Model       name   models CharField          original name   None      def   init   self   args    kwargs           super Person  self    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 Person  self  save force insert  force update   args    kwargs          self   original name   self name

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Note that field change tracking is available in django-model-utils   https   django-model-utils readthedocs org en latest index html

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How about using David Cramer s solution   http   cramer io 2010 12 06 tracking-changes-to-fields-in-django   I ve had success using it like this    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

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improving  josh answer for all fields   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    just to clarify  the getattr works to get fields like person name with strings  i e  getattr person   name

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I have extended the mixin of  livskiy as follows   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   and the DictField is   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    it can be used by extending it in your models a  dict field will be added when you sync migrate and that field will store the state of your objects

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The optimal solution is probably one that does not include an additional database read operation prior to saving the model instance  nor any further django-library  This is why laffuste s solutions is preferable  In the context of an admin site  one can simply override the save model-method  and invoke the form s has changed method there  just as in Sion s answer above  You arrive at something like this  drawing on Sion s example setting but using changed data to get every possible change   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     Override save model    https   docs djangoproject com en 1 10 ref contrib admin  django contrib admin ModelAdmin save model   Built-in changed data-method for a Field    https   docs djangoproject com en 1 10 ref forms api  django forms Form changed data

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If you are using a form  you can use Form s 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

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And now for direct answer  one way to check if the value for the field has changed is to fetch original data from database before saving instance  Consider this example   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    The same thing applies when working with a form  You can detect it at the clean or save method of a ModelForm   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

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I am a bit late to the party but I found this solution also  Django Dirty Fields

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Another late answer  but if you re just trying to see if a new file has been uploaded to a file field  try this   adapted from Christopher Adams s comment on the link http   zmsmith com 2010 05 django-check-if-a-field-has-changed  in zach s comment here   Updated link  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

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This works for me in Django 1 8  def clean self       if self cleaned data  name      self initial  name              Do something

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As of Django 1 8  there s the from db method  as Serge mentions  In fact  the Django docs include this specific use case as an example   https   docs djangoproject com en dev ref models instances  customizing-model-loading     Below is an example showing how to record the initial values of fields that are loaded from the database

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A modification to  ivanperelivskiy s answer    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   This uses django 1 10 s public method get fields instead  This makes the code more future proof  but more importantly also includes foreign keys and fields where editable False   For reference  here is the implementation of  fields   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

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Best way is with a pre save signal  May not have been an option back in  09 when this question was asked and answered  but anyone seeing this today should do it this way    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

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The mixin from  ivanlivski is great    I ve extended it to   Ensure it works with Decimal fields  Expose properties to simplify usage   The updated code is available here  https   github com sknutsonsf python-contrib blob master src django utils ModelDiffMixin py  To help people new to Python or Django  I ll give a more complete example  This particular usage is to take a file from a data provider and ensure the records in the database reflect the file     My model object   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         lt list of many other fields gt      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    gt  0 01   The class that loads the file has these methods   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   lt  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

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There is an attribute   dict   which have all the fields as the keys and value as the field values  So we can just compare two of them  Just change the save function of model to the function below  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    Example Usage   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    yields output with only those fields that have  been changed    name     initial value    1234515    final value    nim     senior     initial value    no    final value    yes

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Here is another way of doing it   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    As per documentation  validating objects   The second step full clean   performs is to call Model clean    This method should be overridden to perform custom validation on your model  This method should be used to provide custom model validation  and to modify attributes on your model if desired  For instance  you could use it to automatically provide a value for a field  or to do validation that requires access to more than a single field

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I use following 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     Usage    gt  gt  gt  p   Place    gt  gt  gt  p has changed False  gt  gt  gt  p changed fields     gt  gt  gt  p rank   42  gt  gt  gt  p has changed True  gt  gt  gt  p changed fields   rank    gt  gt  gt  p diff   rank    0  42    gt  gt  gt  p categories    1  3  5   gt  gt  gt  p diff   categories    None   1  3  5     rank    0  42    gt  gt  gt  p get field diff  categories    None   1  3  5    gt  gt  gt  p get field diff  rank    0  42   gt  gt  gt      Note  Please note that this solution works well in context of current request only  Thus it s suitable primarily for simple cases  In concurrent environment where multiple requests can manipulate the same model instance at the same time  you definitely need a different approach

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You can use django-model-changes to do this without an additional database lookup   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

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I had this situation before my solution was to override the pre save   method of the target field class it will be called only if the field has been changed  useful with FileField example     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   disadvantage  not useful if you want to do any  post save  operation like using the created object in some job  if certain field has changed

[django] When saving, how can you check if a field has changed?

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