#22858 closed New feature (wontfix)

Maximum Recursion Depth Exceeded When Model __init__ Uses Fields Not Included In only(...)

Reported by: pkenjora@… Owned by: nobody
Component: Documentation Version: 1.6
Severity: Normal Keywords: recursion only init
Cc: Triage Stage: Unreviewed
Has patch: no Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: yes UI/UX: no


This line of code throws the error :

[p.date_updated for p in Project.objects.all().only('slug', 'date_updated').select_related('projectimage_set')] \ # REMOVE ONLY AND ERROR GOES AWAY

I've narrowed this down to a specific line in my models.py that triggers the issue as well in combination with the above.

The issue seems to be that when only(...) is used and a function like init references a model field not included in the only list, a recursion error occurs.

class Project(models.Model):
  title = models.CharField(max_length=100, unique=True)
  slug = models.SlugField(blank=False, max_length=100, unique=True, editable=False)
  description = models.CharField(blank=True, max_length=155, editable=False)
  summary = models.TextField(help_text="Describe the project in 3-4 sentences.")

  image_x = models.PositiveIntegerField(blank=True, editable=False, default=1)
  image_y = models.PositiveIntegerField(blank=True, editable=False, default=1)
  image = models.ImageField(upload_to='projects', height_field='image_y', width_field='image_x', storage=S3Storage(bucket=settings.AWS_BUCKET, size=(1000, 1000)), blank=True)

  car = models.ForeignKey(Car, blank=True, null=True)
  parts = models.ManyToManyField(Part);
  faqs = models.ManyToManyField(FAQ, blank=True);

  views = models.PositiveIntegerField(default=0, editable=False, db_index=True)
  sequence = models.PositiveIntegerField(blank=True, default=0)
  date_updated = models.DateField(auto_now=True)

  notify = models.TextField(blank=True, help_text="List of emails seperated by spaces or commas to notify when project is updated.")

  #old_notify = None
  #old_date_updated = None

  class Meta:
    ordering = ['sequence']

  def __unicode__(self):
    return self.title

  def clean(self):
    self.slug = slugify(self.title)

  def __init__(self, *args, **kwargs):
    super(Project, self).__init__(*args, **kwargs)
    self.old_notify = self.notify  # COMMENT THIS LINE OUT AND IT WORKS OK
    self.old_date_updated = self.date_updated

The stack trace is....


Professional Plea: There are some circles who claim using init on a model is not "Pythonic". Neither is a maximum recursion error when you follow "syntax". This use of init is VERY COMMON as a clean quick alternative to signals. If there really is no fix please update the Django documentation to specifically call out that init will throw this issue when used with only(...) to discourage the use of init or at least save people hours of bug hunting.

Change History (5)

comment:1 Changed 18 months ago by timo

  • Needs documentation unset
  • Needs tests unset
  • Patch needs improvement unset
  • Resolution set to wontfix
  • Status changed from new to closed

As documented, "Each deferred field will be retrieved from the database if you access that field (one at a time, not all the deferred fields at once)."

If you access a field in __init__(), you should include it in all of your only() calls to avoid hitting the database again for each item in the queryset. I'm not convinced this is a common enough problem that it needs a special mention, but thanks for the suggestion.

comment:2 Changed 18 months ago by pkenjora@…

Thanks, how about adding a warning like you have to "extra", it seems easy enough to do.


"If you are performing queries on MySQL, note that MySQL’s silent type coercion may cause unexpected results when mixing types. "

"If you use only or defer and use fields outside the set specified, for example in the model init, unexpected errors may occur such as maximum recursion depth."

Seems like the severity and obscurity of the error, although infrequent, warrant a bit more than ignoring the ungraceful failure.

comment:3 Changed 18 months ago by pkenjora

  • Component changed from Database layer (models, ORM) to Documentation
  • Easy pickings set
  • Resolution wontfix deleted
  • Status changed from closed to new
  • Type changed from Bug to New feature

Add warning to only and defer field: If you use only or defer and use fields outside the set specified, for example in the model init, unexpected errors may occur such as maximum recursion depth.

comment:4 Changed 18 months ago by timo

Well, there is now this ticket that people may find via Google if they run into the problem. I'm not in favor of the addition (we cannot document every usage error), but I won't stand in the way if someone else likes it.

comment:5 Changed 15 months ago by timgraham

  • Resolution set to wontfix
  • Status changed from new to closed
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