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Duplicate Django Model Instance and All Foreign Keys Pointing to It

I want to create a method on a Django model, call it model.duplicate(), that duplicates the model instance, including all the foreign keys pointing to it. I know that you can do this:
def duplicate(self):
self.pk = None
self.save()
...but this way all the related models still point to the old instance.
I can't simply save a reference to the original object because what self points to changes during execution of the method:
def duplicate(self):
original = self
self.pk = None
self.save()
assert original is not self # fails
I could try to save a reference to just the related object:
def duplicate(self):
original_fkeys = self.fkeys.all()
self.pk = None
self.save()
self.fkeys.add(*original_fkeys)
...but this transfers them from the original record to the new one. I need them copied over and pointed at the new record.
Several answers elsewhere (and here before I updated the question) have suggested using Python's copy, which I suspect works for foreign keys on this model, but not foreign keys on another model pointing to it.
def duplicate(self):
new_model = copy.deepcopy(self)
new_model.pk = None
new_model.save()
If you do this new_model.fkeys.all() (to follow my naming scheme thus far) will be empty.

You can create new instance and save it like this
def duplicate(self):
kwargs = {}
for field in self._meta.fields:
kwargs[field.name] = getattr(self, field.name)
# or self.__dict__[field.name]
kwargs.pop('id')
new_instance = self.__class__(**kwargs)
new_instance.save()
# now you have id for the new instance so you can
# create related models in similar fashion
fkeys_qs = self.fkeys.all()
new_fkeys = []
for fkey in fkey_qs:
fkey_kwargs = {}
for field in fkey._meta.fields:
fkey_kwargs[field.name] = getattr(fkey, field.name)
fkey_kwargs.pop('id')
fkey_kwargs['foreign_key_field'] = new_instance.id
new_fkeys.append(fkey_qs.model(**fkey_kwargs))
fkeys_qs.model.objects.bulk_create(new_fkeys)
return new_instance
I'm not sure how it'll behave with ManyToMany fields. But for simple fields it works. And you can always pop the fields you are not interested in for your new instance.
The bits where I'm iterating over _meta.fields may be done with copy but the important thing is to use the new id for the foreign_key_field.
I'm sure it's programmatically possible to detect which fields are foreign keys to the self.__class__ (foreign_key_field) but since you can have more of them it'll better to name the one (or more) explicitly.

Although I accepted the other poster's answer (since it helped me get here), I wanted to post the solution I ended up with in case it helps someone else stuck in the same place.
def duplicate(self):
"""
Duplicate a model instance, making copies of all foreign keys pointing
to it. This is an in-place method in the sense that the record the
instance is pointing to will change once the method has run. The old
record is still accessible but must be retrieved again from
the database.
"""
# I had a known set of related objects I wanted to carry over, so I
# listed them explicitly rather than looping over obj._meta.fields
fks_to_copy = list(self.fkeys_a.all()) + list(self.fkeys_b.all())
# Now we can make the new record
self.pk = None
# Make any changes you like to the new instance here, then
self.save()
foreign_keys = {}
for fk in fks_to_copy:
fk.pk = None
# Likewise make any changes to the related model here
# However, we avoid calling fk.save() here to prevent
# hitting the database once per iteration of this loop
try:
# Use fk.__class__ here to avoid hard-coding the class name
foreign_keys[fk.__class__].append(fk)
except KeyError:
foreign_keys[fk.__class__] = [fk]
# Now we can issue just two calls to bulk_create,
# one for fkeys_a and one for fkeys_b
for cls, list_of_fks in foreign_keys.items():
cls.objects.bulk_create(list_of_fks)
What it looks like when you use it:
In [6]: model.id
Out[6]: 4443
In [7]: model.duplicate()
In [8]: model.id
Out[8]: 17982
In [9]: old_model = Model.objects.get(id=4443)
In [10]: old_model.fkeys_a.count()
Out[10]: 2
In [11]: old_model.fkeys_b.count()
Out[11]: 1
In [12]: model.fkeys_a.count()
Out[12]: 2
In [13]: model.fkeys_b.count()
Out[13]: 1
Model and related_model names changed to protect the innocent.

I tried the other answers in Django 2.1/Python 3.6 and they didn't seem to copy one-to-many and many-to-many related objects (self._meta.fields doesn't include one-to-many related fields but self._meta.get_fields() does). Also, the other answers required prior knowledge of the related field name or knowledge of which foreign keys to copy.
I wrote a way to do this in a more generic fashion, handling one-to-many and many-to-many related fields. Comments included, and suggestions welcome:
def duplicate_object(self):
"""
Duplicate a model instance, making copies of all foreign keys pointing to it.
There are 3 steps that need to occur in order:
1. Enumerate the related child objects and m2m relations, saving in lists/dicts
2. Copy the parent object per django docs (doesn't copy relations)
3a. Copy the child objects, relating to the copied parent object
3b. Re-create the m2m relations on the copied parent object
"""
related_objects_to_copy = []
relations_to_set = {}
# Iterate through all the fields in the parent object looking for related fields
for field in self._meta.get_fields():
if field.one_to_many:
# One to many fields are backward relationships where many child objects are related to the
# parent (i.e. SelectedPhrases). Enumerate them and save a list so we can copy them after
# duplicating our parent object.
print(f'Found a one-to-many field: {field.name}')
# 'field' is a ManyToOneRel which is not iterable, we need to get the object attribute itself
related_object_manager = getattr(self, field.name)
related_objects = list(related_object_manager.all())
if related_objects:
print(f' - {len(related_objects)} related objects to copy')
related_objects_to_copy += related_objects
elif field.many_to_one:
# In testing so far, these relationships are preserved when the parent object is copied,
# so they don't need to be copied separately.
print(f'Found a many-to-one field: {field.name}')
elif field.many_to_many:
# Many to many fields are relationships where many parent objects can be related to many
# child objects. Because of this the child objects don't need to be copied when we copy
# the parent, we just need to re-create the relationship to them on the copied parent.
print(f'Found a many-to-many field: {field.name}')
related_object_manager = getattr(self, field.name)
relations = list(related_object_manager.all())
if relations:
print(f' - {len(relations)} relations to set')
relations_to_set[field.name] = relations
# Duplicate the parent object
self.pk = None
self.save()
print(f'Copied parent object ({str(self)})')
# Copy the one-to-many child objects and relate them to the copied parent
for related_object in related_objects_to_copy:
# Iterate through the fields in the related object to find the one that relates to the
# parent model (I feel like there might be an easier way to get at this).
for related_object_field in related_object._meta.fields:
if related_object_field.related_model == self.__class__:
# If the related_model on this field matches the parent object's class, perform the
# copy of the child object and set this field to the parent object, creating the
# new child -> parent relationship.
related_object.pk = None
setattr(related_object, related_object_field.name, self)
related_object.save()
text = str(related_object)
text = (text[:40] + '..') if len(text) > 40 else text
print(f'|- Copied child object ({text})')
# Set the many-to-many relations on the copied parent
for field_name, relations in relations_to_set.items():
# Get the field by name and set the relations, creating the new relationships
field = getattr(self, field_name)
field.set(relations)
text_relations = []
for relation in relations:
text_relations.append(str(relation))
print(f'|- Set {len(relations)} many-to-many relations on {field_name} {text_relations}')
return self

Here is a somewhat simple-minded solution. This does not depend on any undocumented Django APIs. It assumes that you want to duplicate a single parent record, along with its child, grandchild, etc. records. You pass in a whitelist of classes that should actually be duplicated, in the form of a list of names of the one-to-many relationships on each parent object that point to its child objects. This code assumes that, given the above whitelist, the entire tree is self-contained, with no external references to worry about.
One more thing about this code: it is truly recursive, in that it calls itself for each new level of descendants.
from collections import OrderedDict
def duplicate_model_with_descendants(obj, whitelist, _new_parent_pk=None):
kwargs = {}
children_to_clone = OrderedDict()
for field in obj._meta.get_fields():
if field.name == "id":
pass
elif field.one_to_many:
if field.name in whitelist:
these_children = list(getattr(obj, field.name).all())
if children_to_clone.has_key(field.name):
children_to_clone[field.name] |= these_children
else:
children_to_clone[field.name] = these_children
else:
pass
elif field.many_to_one:
if _new_parent_pk:
kwargs[field.name + '_id'] = _new_parent_pk
elif field.concrete:
kwargs[field.name] = getattr(obj, field.name)
else:
pass
new_instance = obj.__class__(**kwargs)
new_instance.save()
new_instance_pk = new_instance.pk
for ky in children_to_clone.keys():
child_collection = getattr(new_instance, ky)
for child in children_to_clone[ky]:
child_collection.add(duplicate_model_with_descendants(child, whitelist=whitelist, _new_parent_pk=new_instance_pk))
return new_instance
Example usage:
from django.db import models
class Book(models.Model)
class Chapter(models.Model)
book = models.ForeignKey(Book, related_name='chapters')
class Page(models.Model)
chapter = models.ForeignKey(Chapter, related_name='pages')
WHITELIST = ['books', 'chapters', 'pages']
original_record = models.Book.objects.get(pk=1)
duplicate_record = duplicate_model_with_descendants(original_record, WHITELIST)

Check if objects is foreign key of queryset result

I have a set of objects stored in a variable called subs. The column subscribed_to is a foreign object.
Is there any way to do check if a related_object is in this list in a simpler way:
def check_subscription_status(user, related_object):
subs = get_user_subscriptions(user) # returns filter queryset
subscribed = False
for s in subs:
if s.subscribed_to == related_object: #related object is the potential match
subscribed = True
break
return subscribed

return (related_object in [s.subscribed_to for s in subs])
ETA: A better way:
return subs.filter(subscribed_to=related_object).count() > 0

how to get entities which don't have certain attribute in datastore

I'm trying to make an appraisal system
This is my class
class Goal(db.Expando):
GID = db.IntegerProperty(required=True)
description = db.TextProperty(required=True)
time = db.FloatProperty(required=True)
weight = db.IntegerProperty(required=True)
Emp = db.UserProperty(auto_current_user=True)
Status = db.BooleanProperty(default=False)
Following things are given by employee,
class SubmitGoal(webapp.RequestHandler):
def post(self):
dtw = simplejson.loads(self.request.body)
try:
maxid = Goal.all().order("-GID").get().GID + 1
except:
maxid = 1
try:
g = Goal(GID=maxid, description=dtw[0], time=float(dtw[1]), weight=int(dtw[2]))
g.put()
self.response.out.write(simplejson.dumps("Submitted"))
except:
self.response.out.write(simplejson.dumps("Error"))
Now, here Manager checks the goals and approve it or not.. if approved then status will be stored as true in datastore else false
idsta = simplejson.loads(self.request.body)
try:
g = db.Query(Goal).filter("GID =", int(idsta[0])).get()
if g:
if idsta[1]:
g.Status=True
try:
del g.Comments
except:
None
else:
g.Status=False
g.Comments=idsta[2]
db.put(g)
self.response.out.write(simplejson.dumps("Submitted"))
except:
self.response.out.write(simplejson.dumps("Error"))
Now, this is where im stuck..."filter('status=',True)".. this is returning all the entities which has status true.. means which are approved.. i want those entities which are approved AND which have not been assessed by employee yet..
def get(self):
t = []
for g in Goal.all().filter("Status = ",True):
t.append([g.GID, g.description, g.time, g.weight, g.Emp])
self.response.out.write(simplejson.dumps(t))
def post(self):
idasm = simplejson.loads(self.request.body)
try:
g = db.Query(Goal).filter("GID =", int(idasm[0])).get()
if g:
g.AsmEmp=idasm[1]
db.put(g)
self.response.out.write(simplejson.dumps("Submitted"))
except:
self.response.out.write(simplejson.dumps("Error"))
How am I supposed to do this? as I know that if I add another filter like "filter('AsmEmp =', not None)" this will only return those entities which have the AsmEmp attribute what I need is vice versa.

You explicitly can't do this. As the documentation states:
It is not possible to query for entities that are missing a given property.
Instead, create a property for is_assessed which defaults to False, and query on that.

could you not simply add another field for when employee_assessed = db.user...
and only populate this at the time when it is assessed?

The records do not lack the attribute in the datastore, it's simply set to None. You can query for those records with Goal.all().filter('status =', True).filter('AsmEmp =', None).
A few incidental suggestions about your code:
'Status' is a rather unintuitive name for a boolean.
It's generally good Python style to begin properties and attributes with a lower-case letter.
You shouldn't iterate over a query directly. This fetches results in batches, and is much less efficient than doing an explicit fetch. Instead, fetch the number of results you need with .fetch(n).
A try/except with no exception class specified and no action taken when an exception occurs is a very bad idea, and can mask a wide variety of issues.
Edit: I didn't notice that you were using an Expando - in which case #Daniel's answer is correct. There doesn't seem to be any good reason to use Expando here, though. Adding the property to the model (and updating existing entities) would be the easiest solution here.

Copy an entity in Google App Engine datastore in Python without knowing property names at 'compile' time

In a Python Google App Engine app I'm writing, I have an entity stored in the datastore that I need to retrieve, make an exact copy of it (with the exception of the key), and then put this entity back in.
How should I do this? In particular, are there any caveats or tricks I need to be aware of when doing this so that I get a copy of the sort I expect and not something else.
ETA: Well, I tried it out and I did run into problems. I would like to make my copy in such a way that I don't have to know the names of the properties when I write the code. My thinking was to do this:
#theThing = a particular entity we pull from the datastore with model Thing
copyThing = Thing(user = user)
for thingProperty in theThing.properties():
copyThing.__setattr__(thingProperty[0], thingProperty[1])
This executes without any errors... until I try to pull copyThing from the datastore, at which point I discover that all of the properties are set to None (with the exception of the user and key, obviously). So clearly this code is doing something, since it's replacing the defaults with None (all of the properties have a default value set), but not at all what I want. Suggestions?

Here you go:
def clone_entity(e, **extra_args):
"""Clones an entity, adding or overriding constructor attributes.
The cloned entity will have exactly the same property values as the original
entity, except where overridden. By default it will have no parent entity or
key name, unless supplied.
Args:
e: The entity to clone
extra_args: Keyword arguments to override from the cloned entity and pass
to the constructor.
Returns:
A cloned, possibly modified, copy of entity e.
"""
klass = e.__class__
props = dict((k, v.__get__(e, klass)) for k, v in klass.properties().iteritems())
props.update(extra_args)
return klass(**props)
Example usage:
b = clone_entity(a)
c = clone_entity(a, key_name='foo')
d = clone_entity(a, parent=a.key().parent())
EDIT: Changes if using NDB
Combining Gus' comment below with a fix for properties that specify a different datastore name, the following code works for NDB:
def clone_entity(e, **extra_args):
klass = e.__class__
props = dict((v._code_name, v.__get__(e, klass)) for v in klass._properties.itervalues() if type(v) is not ndb.ComputedProperty)
props.update(extra_args)
return klass(**props)
Example usage (note key_name becomes id in NDB):
b = clone_entity(a, id='new_id_here')
Side note: see the use of _code_name to get the Python-friendly property name. Without this, a property like name = ndb.StringProperty('n') would cause the model constructor to raise an AttributeError: type object 'foo' has no attribute 'n'.

If you're using the NDB you can simply copy with:
new_entity.populate(**old_entity.to_dict())

This is just an extension to Nick Johnson's excellent code to address the problems highlighted by Amir in the comments:
The db.Key value of the ReferenceProperty is no longer retrieved via an unnecessary roundtrip to the datastore.
You can now specify whether you want to skip DateTime properties with the auto_now and/or auto_now_add flag.
Here's the updated code:
def clone_entity(e, skip_auto_now=False, skip_auto_now_add=False, **extra_args):
"""Clones an entity, adding or overriding constructor attributes.
The cloned entity will have exactly the same property values as the original
entity, except where overridden. By default it will have no parent entity or
key name, unless supplied.
Args:
e: The entity to clone
skip_auto_now: If True then all DateTimeProperty propertes will be skipped which have the 'auto_now' flag set to True
skip_auto_now_add: If True then all DateTimeProperty propertes will be skipped which have the 'auto_now_add' flag set to True
extra_args: Keyword arguments to override from the cloned entity and pass
to the constructor.
Returns:
A cloned, possibly modified, copy of entity e.
"""
klass = e.__class__
props = {}
for k, v in klass.properties().iteritems():
if not (type(v) == db.DateTimeProperty and ((skip_auto_now and getattr(v, 'auto_now')) or (skip_auto_now_add and getattr(v, 'auto_now_add')))):
if type(v) == db.ReferenceProperty:
value = getattr(klass, k).get_value_for_datastore(e)
else:
value = v.__get__(e, klass)
props[k] = value
props.update(extra_args)
return klass(**props)
The first if expression is not very elegant so I appreciate if you can share a better way to write it.

I'm neither Python nor AppEngine guru, but couldn't one dynamically get/set the properties?
props = {}
for p in Thing.properties():
props[p] = getattr(old_thing, p)
new_thing = Thing(**props).put()

A variation inspired in Nick's answer which handles the case in which your entity has a (repeated) StructuredProperty, where the StructuredProperty itself has ComputedProperties. It can probably be written more tersely with dict comprehension somehow, but here is the longer version that worked for me:
def removeComputedProps(klass,oldDicc):
dicc = {}
for key,propertType in klass._properties.iteritems():
if type(propertType) is ndb.StructuredProperty:
purged = []
for item in oldDicc[key]:
purged.append(removeComputedProps(propertType._modelclass,item))
dicc[key]=purged
else:
if type(propertType) is not ndb.ComputedProperty:
dicc[key] = oldDicc[key]
return dicc
def cloneEntity(entity):
oldDicc = entity.to_dict()
klass = entity.__class__
dicc = removeComputedProps(klass,oldDicc)
return klass(**dicc)

This can be tricky if you've renamed the underlying keys for your properties... which some people opt to do instead of making mass data changes
say you started with this:
class Person(ndb.Model):
fname = ndb.StringProperty()
lname = ndb.StringProperty()
then one day you really decided that it would be nicer to use first_name and last_name instead... so you do this:
class Person(ndb.Model):
first_name = ndb.StringProperty(name="fname")
last_name = ndb.StringProperty(name="lname")
now when you do Person._properties (or .properties() or person_instance._properties) you will get a dictionary with keys that match the underlying names (fname and lname)... but won't match the actual property names on the class... so it won't work if you put them into the constructor of a new instance, or use the .populate() method (the above examples will break)
In NDB anyways, instances of models have ._values dictionary which is keyed by the underlying property names... and you can update it directly. I ended up with something like this:
def clone(entity, **extra_args):
klass = entity.__class__
clone = klass(**extra_args)
original_values = dict((k,v) for k,v in entity._values.iteritems() if k not in clone._values)
clone._values.update(original_values)
return clone
This isn't really the safest way... as there are other private helper methods that do more work (like validation and conversion of computed properties by using _store_value() and _retrieve_value())... but if you're models are simple enough, and you like living on the edge :)

Here's the code provided by #zengabor with the if expression formatted for easier reading. It may not be PEP-8 compliant:
klass = e.__class__
props = {}
for k, v in klass.properties().iteritems():
if not (type(v) == db.DateTimeProperty and ((
skip_auto_now and getattr(v, 'auto_now' )) or (
skip_auto_now_add and getattr(v, 'auto_now_add')))):
if type(v) == db.ReferenceProperty:
value = getattr(klass, k).get_value_for_datastore(e)
else:
value = v.__get__(e, klass)
props[k] = value
props.update(extra_args)
return klass(**props)

Django model ON DELETE CASCADE policy, emulate ON DELETE RESTRICT instead, general solution

I'd like to delete an instance of a model, but only if it doesn't have any other instance of another class with a foreign key pointing to it. From Django documentation:
When Django deletes an object, it emulates the behavior of the SQL constraint ON DELETE CASCADE -- in other words, any objects which had foreign keys pointing at the object to be deleted will be deleted along with it.
In a given example:
class TestA(models.Model)
name = models.CharField()
class TestB(models.Model)
name = models.CharField()
TestAs = models.ManyToManyField(TestA)
# More classes with a ManyToMany relationship with TestA
# ........
I'd like something like:
tA = TestA(name="testA1")
tB = TestB(name="testB1")
tB.testAs.add(tA)
t = TestA.objects.get(name="testA1")
if is_not_foreignkey(t):
t.delete()
else:
print "Error, some instance is using this"
Should print the error. I know I can check for specific instances the foreignkey sets, like in this case check t.TestB_set(), but I am looking for a more general solution for any given model.

I finally solved it using this Nullable ForeignKeys and deleting a referenced model instance, the solution looks like:
# Check foreign key references
instances_to_be_deleted = CollectedObjects()
object._collect_sub_objects(instances_to_be_deleted)
# Count objects to delete
count_instances_to_delete = 0
for k in instances_to_be_deleted.unordered_keys():
count_instances_to_delete += len(instances_to_be_deleted[k])
if count_instances_to_delete == 1:
object.delete()
else:
pass

Check the related objects length
t=TestA.objects.get(name="textA1")
if not t.testB_set.all().count():#related members
t.delete()

CollectedObjects() was removed in Django 1.3 -- here's a current method:
from compiler.ast import flatten
from django.db import DEFAULT_DB_ALIAS
from django.contrib.admin.util import NestedObjects
def delete_obj_if_no_references(obj):
collector = NestedObjects(using=DEFAULT_DB_ALIAS)
collector.collect([obj])
objs = flatten(collector.nested())
if len(objs) == 1 and objs[0] is obj:
obj.delete()
return True
return False