[Solved]-Limit a single record in model for django app?

Try this:

class MyModel(models.Model):
    onefield = models.CharField('The field', max_length=100)

class MyModelAdmin(admin.ModelAdmin):
  def has_add_permission(self, request):
    # if there's already an entry, do not allow adding
    count = MyModel.objects.all().count()
    if count == 0:
      return True

    return False

William is right, but I guess this is the best practice

def has_add_permission(self, *args, **kwargs):
    return not MyModel.objects.exists()

As reported in the official Django Documentation:

Note: Don’t use this if all you want to do is determine if at least one result exists.
It’s more efficient to use exists().

https://docs.djangoproject.com/en/dev/ref/models/querysets/#when-querysets-are-evaluated

Overwriting has_add_permission works, but in the given examples it breaks the permissions system in Django(staff without necessary permissions can add settings). Here’s a one that doesn’t break it:

class SettingAdmin(admin.ModelAdmin):
    def has_add_permission(self, request):
        base_add_permission = super(SettingAdmin, self).has_add_permission(request)
        if base_add_permission:
            # if there's already an entry, do not allow adding
            count = Setting.objects.all().count()
            if count == 0:
                return True
        return False

A model with a single allowed row is nothing more than a perverted form of a “persisted object” — maybe even a “persisted singleton”? Don’t do that, that’s not how models work.

Check out https://github.com/danielroseman/django-dbsettings

It looks like Ali Reza’s answer but you can update the existed records and return the error message to any form that uses this model. I believe it is reliable and much easy to control.

class MyModel(models.Model):
    ...

    def clean(self):
        super().clean()
        if not self.id and MyModel.objects.exists():
            raise ValidationError('You cannot add more somethings.')

The following is a class I have created which can be used as a singleton class.

from django.db import models
class SingletonModel(models.Model):
    class Meta:
        abstract = True

    def save(self, *args, **kwargs):
        self.__class__.objects.exclude(id=self.id).delete()
        super(SingletonModel, self).save(*args, **kwargs)

    @classmethod
    def load(cls):
        try:
            return cls.objects.get()
        except cls.DoesNotExist:
            return cls()

From the above SingletonModel we can create multiple models, all of which will be having only one record

class ProjectSettings(SingletonModel):
    max_tickets = models.IntegerField(default=15)
    min_tickets = models.IntegerField(default=2)
    ...

We can access the only object of the settings model as follows

ProjectSettings.load().max_tickets

It is also possible to register ProjectSettings to django admin

@admin.register(ProjectSettings)
class ProjectSettingsAdmin(admin.ModelAdmin):
    list_display = [field.name for field in ProjectSettings._meta.get_fields()]
    def has_delete_permission(self, request, obj=None):
        # Nobody is allowed to delete
        return False

You can rewrite the save method on your model. Whenever the user wants to register a new record, you can check the existence then, either save the record or, reject the request.

class MyModel(models.Model):
    title = models.CharField(...)
    body = models.TextField(...)
    
    def save(self, *args, **kwargs):
        if MyModel.objects.exists():
            raise ValueError("This model has already its record.")
        else:
            super().save(*args, **kwargs)

You can also use validators. I prefer to use this method.
Hope you find it useful.