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Unique BooleanField value in Django?

Posted by: admin November 30, 2017 Leave a comment

Questions:

Suppose my models.py is like so:

class Character(models.Model):
    name = models.CharField(max_length=255)
    is_the_chosen_one = models.BooleanField()

I want only one of my Character instances to have is_the_chosen_one == True and all others to have is_the_chosen_one == False . How can I best ensure this uniqueness constraint is respected?

Top marks to answers that take into account the importance of respecting the constraint at the database, model and (admin) form levels!

Answers:

Whenever I’ve needed to accomplish this task, what I’ve done is override the save method for the model and have it check if any other model has the flag already set (and turn it off).

class Character(models.Model):
    name = models.CharField(max_length=255)
    is_the_chosen_one = models.BooleanField()

    def save(self, *args, **kwargs):
        if self.is_the_chosen_one:
            try:
                temp = Character.objects.get(is_the_chosen_one=True)
                if self != temp:
                    temp.is_the_chosen_one = False
                    temp.save()
            except Character.DoesNotExist:
                pass
        super(Character, self).save(*args, **kwargs)

Questions:
Answers:

Instead of using custom model cleaning/saving, I created a custom field overriding the pre_save method on django.db.models.BooleanField. Instead of raising an error if another field was True, I made all other fields False if it was True. Also instead of raising an error if the field was False and no other field was True, I saved it the field as True

fields.py

from django.db.models import BooleanField


class UniqueBooleanField(BooleanField):
    def pre_save(self, model_instance, add):
        objects = model_instance.__class__.objects
        # If True then set all others as False
        if getattr(model_instance, self.attname):
            objects.update(**{self.attname: False})
        # If no true object exists that isnt saved model, save as True
        elif not objects.exclude(id=model_instance.id)\
                        .filter(**{self.attname: True}):
            return True
        return getattr(model_instance, self.attname)

# To use with South
from south.modelsinspector import add_introspection_rules
add_introspection_rules([], ["^project\.apps\.fields\.UniqueBooleanField"])

models.py

from django.db import models

from project.apps.fields import UniqueBooleanField


class UniqueBooleanModel(models.Model):
    unique_boolean = UniqueBooleanField()

    def __unicode__(self):
        return str(self.unique_boolean)

Questions:
Answers:

I’d override the save method of the model and if you’ve set the boolean to True, make sure all others are set to False.

from django.db import transaction

class Character(models.Model):
    name = models.CharField(max_length=255)
    is_the_chosen_one = models.BooleanField()

    @transaction.atomic
    def save(self, *args, **kwargs):
        if self.is_the_chosen_one:
            Character.objects.filter(
                is_the_chosen_one=True).update(is_the_chosen_one=False)
        super(Character, self).save(*args, **kwargs)

I tried editing the similar answer by Adam, but it was rejected for changing too much of the original answer. This way is more succinct and efficient as the checking of other entries is done in a single query.

Questions:
Answers:
class Character(models.Model):
    name = models.CharField(max_length=255)
    is_the_chosen_one = models.BooleanField()

    def save(self, *args, **kwargs):
        if self.is_the_chosen_one:
            qs = Character.objects.filter(is_the_chosen_one=True)
            if self.pk:
                qs = qs.exclude(pk=self.pk)
            if qs.count() != 0:
                # choose ONE of the next two lines
                self.is_the_chosen_one = False # keep the existing "chosen one"
                #qs.update(is_the_chosen_one=False) # make this obj "the chosen one"
        super(Character, self).save(*args, **kwargs)

class CharacterForm(forms.ModelForm):
    class Meta:
        model = Character

    # if you want to use the new obj as the chosen one and remove others, then
    # be sure to use the second line in the model save() above and DO NOT USE
    # the following clean method
    def clean_is_the_chosen_one(self):
        chosen = self.cleaned_data.get('is_the_chosen_one')
        if chosen:
            qs = Character.objects.filter(is_the_chosen_one=True)
            if self.instance.pk:
                qs = qs.exclude(pk=self.instance.pk)
            if qs.count() != 0:
                raise forms.ValidationError("A Chosen One already exists! You will pay for your insolence!")
        return chosen

You can use the above form for admin as well, just use

class CharacterAdmin(admin.ModelAdmin):
    form = CharacterForm
admin.site.register(Character, CharacterAdmin)

Questions:
Answers:
class Character(models.Model):
    name = models.CharField(max_length=255)
    is_the_chosen_one = models.BooleanField()

    def clean(self):
        from django.core.exceptions import ValidationError
        c = Character.objects.filter(is_the_chosen_one__exact=True)  
        if c and self.is_the_chosen:
            raise ValidationError("The chosen one is already here! Too late")

Doing this made the validation available in the basic admin form

Questions:
Answers:

The following solution is a little bit ugly but might work:

class MyModel(models.Model):
    is_the_chosen_one = models.NullBooleanField(default=None, unique=True)

    def save(self, *args, **kwargs):
        if self.is_the_chosen_one is False:
            self.is_the_chosen_one = None
        super(MyModel, self).save(*args, **kwargs)

If you set is_the_chosen_one to False or None it will be always NULL. You can have NULL as much as you want, but you can only have one True.

Questions:
Answers:

Trying to make ends meet with the answers here, I find that some of them address the same issue successfully and each one is suitable in different situations:

I would choose:

  • @semente: Respects the constraint at the database, model and admin form levels while it overrides Django ORM the least possible. Moreover it can probably be used inside a through table of a ManyToManyField in aunique_together situation. (I will check it and report)

    class MyModel(models.Model):
        is_the_chosen_one = models.NullBooleanField(default=None, unique=True)
    
        def save(self, *args, **kwargs):
            if self.is_the_chosen_one is False:
                self.is_the_chosen_one = None
            super(MyModel, self).save(*args, **kwargs)
    
  • @Flyte: Hits the database only one extra time and accepts the current entry as the chosen one. Clean and elegant.

    from django.db import transaction
    
    class Character(models.Model):
        name = models.CharField(max_length=255)
        is_the_chosen_one = models.BooleanField()
    
        @transaction.atomic
        def save(self, *args, **kwargs):
            if self.is_the_chosen_one:
                Character.objects.filter(
                    is_the_chosen_one=True).update(is_the_chosen_one=False)
            super(Character, self).save(*args, **kwargs)
    

Other solutions not suitable for my case but viable:

@nemocorp is overriding the clean method to perform a validation. However, it does not report back which model is “the one” and this is not user friendly. Despite that, it is a very nice approach especially if someone does not intend to be as aggressive as @Flyte.

@saul.shanabrook and @Thierry J. would create a custom field which would either change any other “is_the_one” entry to False or raise a ValidationError. I am just reluctant to impement new features to my Django installation unless it is absoletuly necessary.

@daigorocub: Uses Django signals. I find it a unique approach and gives a hint of how to use Django Signals. However I am not sure whether this is a -strictly speaking- “proper” use of signals since I cannot consider this procedure as part of a “decoupled application”.

Questions:
Answers:

And that’s all.

def save(self, *args, **kwargs):
    if self.default_dp:
        DownloadPageOrder.objects.all().update(**{'default_dp': False})
    super(DownloadPageOrder, self).save(*args, **kwargs)

Questions:
Answers:

I tried some of these solutions, and ended up with another one, just for the sake of code shortness (don’t have to override forms or save method).
For this to work, the field can’t be unique in it’s definition but the signal makes sure that happens.

# making default_number True unique
@receiver(post_save, sender=Character)
def unique_is_the_chosen_one(sender, instance, **kwargs):
    if instance.is_the_chosen_one:
        Character.objects.all().exclude(pk=instance.pk).update(is_the_chosen_one=False)

Questions:
Answers:

Using a similar approach as Saul, but slightly different purpose:

class TrueUniqueBooleanField(BooleanField):

    def __init__(self, unique_for=None, *args, **kwargs):
        self.unique_for = unique_for
        super(BooleanField, self).__init__(*args, **kwargs)

    def pre_save(self, model_instance, add):
        value = super(TrueUniqueBooleanField, self).pre_save(model_instance, add)

        objects = model_instance.__class__.objects

        if self.unique_for:
            objects = objects.filter(**{self.unique_for: getattr(model_instance, self.unique_for)})

        if value and objects.exclude(id=model_instance.id).filter(**{self.attname: True}):
            msg = 'Only one instance of {} can have its field {} set to True'.format(model_instance.__class__, self.attname)
            if self.unique_for:
                msg += ' for each different {}'.format(self.unique_for)
            raise ValidationError(msg)

        return value

This implementation will raise a ValidationError when attempting to save another record with a value of True.

Also, I have added the unique_for argument which can be set to any other field in the model, to check true-uniqueness only for records with the same value, such as:

class Phone(models.Model):
    user = models.ForeignKey(User)
    main = TrueUniqueBooleanField(unique_for='user', default=False)

Questions:
Answers:

Do I get points for answering my question?

problem was it was finding itself in the loop, fixed by:

    # is this the testimonial image, if so, unselect other images
    if self.testimonial_image is True:
        others = Photograph.objects.filter(project=self.project).filter(testimonial_image=True)
        pdb.set_trace()
        for o in others:
            if o != self: ### important line
                o.testimonial_image = False
                o.save()