I ran into the same problem today in a script for creating large amounts of
test data. It would be awkward to re-organize the code it such a way that
related objects are assigned to fields only after they've been saved, so I
made a utility method to a take a model instance which has fields which
might be assigned related objects before they are saved, and "fix" those
fields after the related objects have been saved:
def _resync_fk_fields(self, obj):
for field in type(obj)._meta.local_concrete_fields:
if isinstance(field, models.ForeignKey):
rel_obj = getattr(obj, field.name)
if rel_obj is not None:
setattr(obj, field.get_attname(), rel_obj.id)
Seems to do the trick
-Rowan
On Monday, 28 November 2016 16:54:41 UTC+2, Michael Viveros wrote:
>
> Thanks for the detailed responses, they helped clarify things a lot.
>
> I will try the first option you mentioned about creating the Questions
> first (and saving them) and then creating the Choices using the Questions
> that were just created.
> My back-up plan will be to use a different pk field for Question (like
> question text) that will be already known when I create Choices for a
> question.
>
> On Monday, November 28, 2016 at 8:55:02 AM UTC-5, C. Kirby wrote:
>>
>>
>>
>> On Monday, November 28, 2016 at 3:12:37 PM UTC+2, Michael Viveros wrote:
>>>
>>> Thanks for the reply Kirby.
>>>
>>> I understand that the Question's pk is null when you pass in the
>>> Question to the Choice constructor. But when you save the Choice, the
>>> Question was saved earlier so the Question's pk is not null. But the
>>> Choice's question_id attribute (representing the Question's pk) is still
>>> null when you save the Choice.
>>>
>>> *1. Should the Choice's question_id have been updated to it's current
>>> value when saving the Choice?*
>>>
>> No. The preparing you linked handles type changes between python and the
>> db, if the db cannot handle the type you have (like the referenced python
>> datetime).
>> What is happening is no different than:
>>
>> >>> a = 1
>> >>> b = a
>> >>> a = 2
>> >>> b
>> 1
>> >>> a
>> 2
>>
>>
>>> I read some more about what happens when you save
>>> <https://docs.djangoproject.com/en/1.10/ref/models/instances/#what-happens-when-you-save>
>>> an
>>> object in Django and it seems like the Choice's question_id should be
>>> updated when the data is prepared for the db (step 3 from the link).
>>> I understand I can override the Choice class's save method to do
>>> this (see code below) but it seems like it should be something that is
>>> already done by the Django framework.
>>>
>>> def save(self, *args, **kwargs):
>>> self.question_id = self.question.id
>>> super(Choice, self).save(*args, **kwargs)
>>>
>>>
>>> This won't work for 1 or 2 reasons, depending on versions. save() does
>> not get called when using bulk_create(). Also, unless you are using
>> postgres and django 1.10, pk's get saved to the db, but not assigned to the
>> object when using bulk_create()
>>
>>> *2. Is there a better way to bulk create objects in a one-to-many
>>> relationship *(Questions and Choices)*?*
>>>
>>> My idea (from the code snippet after models.py) was to bulk create the
>>> Questions and then bulk create the Choices but this has the problem
>>> mentioned above of the Choice's FK (question_id) not being updated even
>>> after the Questions were bulk created.
>>>
>> You pretty much can't do it inline like you want with bulk_create(), I
>> don't think. If you have the question objects retrieved you can then assign
>> them to the choices and you are all set. If you really want to do it how
>> you are trying then your best bet would be to assign a primary_key field
>> instead of using the AutoField primary key provided. If you use the
>> question title, or generate a uuid for each question and set that as the
>> primary_field then you can provide the primary key that you know for a
>> question to the associated choices.
>>
>>>
>>> On Monday, November 28, 2016 at 6:59:13 AM UTC-5, C. Kirby wrote:
>>>>
>>>> The reason it isn't working in your first example is because the pk
>>>> gets assigned to the object when it is saved to the database. When you
>>>> supply question=q to the choice, it is (correctly) getting a pk of null
>>>> because q has not been saved.
>>>>
>>>> On Saturday, November 26, 2016 at 2:56:37 AM UTC+2, Michael Viveros
>>>> wrote:
>>>>>
>>>>> I have a Question model and a Choice model where a Question can have
>>>>> many Choices so the Choice model has a Foreign Key mapping to a Question.
>>>>>
>>>>> models.py:
>>>>>
>>>>> from django.db import models
>>>>>
>>>>>
>>>>> class Question(models.Model):
>>>>> text = models.CharField(max_length=200)
>>>>> title = models.CharField(max_length=200)
>>>>> total_response_count = models.IntegerField()
>>>>> topic = models.CharField(max_length=20)
>>>>> sequence_num = models.IntegerField()
>>>>> select_multiple = models.BooleanField(default=False)
>>>>>
>>>>> def __str__(self):
>>>>> return self.text
>>>>>
>>>>>
>>>>> class Choice(models.Model):
>>>>> question = models.ForeignKey(Question, on_delete=models.CASCADE)
>>>>> text = models.CharField(max_length=200)
>>>>> response_count = models.IntegerField()
>>>>> response_percent = models.FloatField()
>>>>>
>>>>> def __str__(self):
>>>>> return self.text
>>>>>
>>>>>
>>>>> I need to parse a csv file containing these models and insert them
>>>>> into a database.
>>>>> I want to be efficient and bulk create these models instead of having
>>>>> to call save() after creating each model.
>>>>> I can bulk create the Questions but then when I try to bulk create the
>>>>> Choices I get an IntegrityError about the Choice's foreign key id
>>>>> attribute
>>>>> ("question_id") violating a not-null constraint.
>>>>> After creating the Questions, the Question's id attribute gets updated
>>>>> but the Choice's question_id attribute does not get updated and it does
>>>>> not
>>>>> get re-evaluated when the Choice is created.
>>>>>
>>>>> Here's a simplified version of what I want to do:
>>>>> from censustest.models import Question,Choice
>>>>>
>>>>>
>>>>> q=Question(text="Q1",title="",topic="",sequence_num=0,total_response_count=0)
>>>>> c=Choice(text="C1",response_count=5,response_percent=0.3,question=q)
>>>>> print("q.id %s, c.question %s, c.question_id %s" % (q.id, c.question,
>>>>> c.question_id)) # None,Q1,None
>>>>> Question.objects.bulk_create([q])
>>>>> print("After creating q: q.id %s, c.question %s, c.question_id %s" % (
>>>>> q.id, c.question, c.question_id)) # valid id,Q1,None
>>>>> Choice.objects.bulk_create([c]) # ERROR: IntegrityError
>>>>>
>>>>> Output:
>>>>>
>>>>> <https://lh3.googleusercontent.com/-FFbndWx_lfU/WDjbHC_0QUI/AAAAAAAACPQ/xnwdyg_mVNM6lS-jytZTN7n0ujR7FzYiwCLcB/s1600/Screen%2BShot%2B2016-11-25%2Bat%2B7.42.20%2BPM.png>
>>>>>
>>>>> *Is this a bug with Django *(the foreign key id attribute not being
>>>>> updated)* or is there a better way to do it?*
>>>>>
>>>>> An alternative approach I thought of was to assign the Choice's
>>>>> Question after creating the Questions and this works.
>>>>> from censustest.models import Question,Choice
>>>>>
>>>>>
>>>>> q=Question(text="Q1",title="",topic="",sequence_num=0,total_response_count=0)
>>>>> c=Choice(text="C1",response_count=5,response_percent=0.3)
>>>>> print("q.id %s" % (q.id)) # None,Q1,None
>>>>> Question.objects.bulk_create([q])
>>>>> c.question=q
>>>>> print("c.question %s, c.question_id %s" % (c.question, c.question_id))
>>>>> # Q1,valid id
>>>>> Choice.objects.bulk_create([c]) # works, no error
>>>>>
>>>>> Output:
>>>>>
>>>>> <https://lh3.googleusercontent.com/-pncM3xKl0Gc/WDjboxPCzWI/AAAAAAAACPU/V4I9TczbqnAm6mBnogU-kIVCvWXJ7ebrQCLcB/s1600/Screen%2BShot%2B2016-11-25%2Bat%2B7.43.53%2BPM.png>
>>>>>
>>>>
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