How to check for uniqueness in the derived field of model in Django -

i have model like

class infofromfile(models.model):     name = models.charfield()     title = models.charfield(max_length=200)     date_from_file = models.datefield()     file = models.filefield()     file_hash = models.charfield(unique=true) 

with corresponding admin class

class infofromfileadmin(admin.modeladmin):     fieldsets = [         ('information source', {'fields': ['file']}),     ]      def save_model(self, request, obj, form, change):         # custom action here extract info file         # before creating , saving model 

i have separate function creating hash of file.

so, input field in form file upload. want prevent duplicate files uploaded @ all, is, before saving model part of form/model validation. if model attempted saved , hash exists, integrityerror thrown.

i know model.validate_unique() method, don't know how apply in modeladmin.save_model(*) method.

to summarize, question is:
how can validate uniqueness in different field 1 provided model form? should make use of javascript create hash of file before admin-user hits save button? or make use of other hidden fields?

edit: future expansion site allow users upload such files (or same processing resulting in output pdf-file, possibly without saving user's file), albeit not needed now.

i think need custom form. can validate whatever want in clean_<field> method of form. should similar this:

class infofromfileform(modelform):     def clean_file(self):         file_obj = self.cleaned_data['file']         hash = get_hash() # whatever use file hash         if infofromfile.objects.filter(file_hash=hash).exists():             raise validationerror(_("the hash must unique.")         return file_obj 

then in admin class

class infofromfileadmin(admin.modeladmin):     # things     form = infofromfileform     # more things