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How to allow boolean True on just one model in table in Django?

I've got a model where I would like there to be able to have one set as the 'app default'.
In this model I added a field named is_app_default in order to help accommodate this.
class TreeLevel(models.Model):
id = models.BigAutoField(primary_key=True)
short_description = models.CharField(max_length=200)
long_description = models.TextField()
is_app_default = models.BooleanField(default=False)
class Meta:
verbose_name = "Tree Level"
verbose_name_plural = "Tree Levels"
class Layer(models.Model):
id = models.BigAutoField(primary_key=True)
tree_levels = models.ManyToManyField(TreeLevel)
description = models.TextField()
class Meta:
verbose_name = "Layer"
verbose_name_plural = "Layers"
The Layer model links to TreeLevel with an m2m. Ultimately I would like the is_app_default TreeLevel automatically added to every Layer m2m relationship - which is why there can only be one TreeLevel with is_app_default set as True.
My potential solution(s):
Users with admin may be creating new TreeLevel objects - so I need to make sure they aren't setting that Boolean in any new models. I think I can override the save() method on TreeLevel to check the DB to see if another with that boolean as True exists - if so? Don't save the 'new' one and return an error. But I think this hits the database - causing unnecessary queries potentially?
Then additionally, I would also need to override the save() method on Layer and add the 'default' TreeLevel to that many2many at that time.
Is there a good way to 'automatically' handle this with these relationships?
My ultimate goal:
Have a default TreeLevel that is automatically added to every Layer many to many relationship whenever a Layer is created.
Questions:
Would my proposed solution work?
I've tried looking into Djangos Unique Constraints, but I think those are more on a row level, as opposed to a Table level.

Django intermediary model

I dont understand Django Intermediary models at all. Let's say that i want to add one more additional field to User model, and store relationship between two users in another model(intermediary) like this:
class Contact(models.Model):
user_from = models.ForeignKey(User...)
user_to = models.ForeignKey(User...)
...
And the field i want to add is:
following = models.ManyToManyField('self',
through=Contact,
related_name='followers',
symetrical=False)
What really happen when i call user_object.followers.all() and what is the difference between user_object.following.all() are they both the same?

Convert join SQL query into django

I am new to django and trying out stuff with it.
How do I display selected fields from the joined table.
For example:
I have two models, X and Y. I am merging these two models based on the foreign key of model Y.
class X(models.Model):
name = models.CharField()
id = models.AutoField(primary_key=True)
class Y(models.Model):
owner_user = models.ForeignKey(X, models.DO_NOTHING,
db_column='id')
detail = models.CharField()
How do I write this query as a django code?
SELECT name, id, Body_details
FROM X, Y
WHERE X.id = Y.OwnerUserId;

You can use select_related
a = Y.objects.select_related('OwnerUserId').all()
for object in a:
print(object.OwneruserId.name, object.OwneruserId.id, object.body)

You can make use of select_related here.
result = Y.objects.select_related('owner_use')
All the work behind joining will automatically be done by this ORM using select_related. You can see previously asked questions similar to this one here.

You need to use the related_name of the ForeignKey field, which is y_set by default, to access the reverse relationship of model :
some_id = 1
instance = X.objects.get(id=some_id)
instance.y_set.all()

Fast filter on related fields in django

I have 2 models in my django project.
ModelA(models.Model):
id = models.AutoField(primary_key=True)
field1 = ...
~
fieldN = ...
ModelB(models.Model):
id = models.AutoField(primary_key=True)
a = models.ForeignKey(A, on_delete=models.CASCADE)
field1 = ...
~
fieldN = ...
Here I have one-to-mane relation A->B. Table A has around 30 different fields and 10.000+ rows and table B has around 15 and 10.000.000+ rows. I need to filter firstly by the several ModelA fields and then for each of the filtered ModelA row/object get related ModelB objects and filter them by several fields. After that I need to serialize them in JSON where all ModelB packed in one field as array.
Is it possible to perform this around the 1-3 second? If yes, what is the best approach?
I use PostgreSQL.
EDIT:
Now I am doing it like chain .filter() on simple ModelA fields and then iterate over resulted QuerySet and get set of ModelB for each ModelA instance,but i suspect, that the second part of this solution will slow down whole process, so I suppose there is a better way to do it.

It may be faster to do a query like this:
model_a_queryset = ModelA.objects.filter(field=whatever)
model_b_queryset = ModelB.objects.filter(a__in=model_a_queryset)
Because Django does lazy queryset evaulation, this will only result in one hit to the database.
As an aside, there is no need to define id = Autofield fields on your models. Django includes them by default.

Django: meta class order ignoring NULL

I need to sort queryset using Django model's meta class.
I've defined it like this:
class MyModel(models.Model):
order = models.IntegerField(blank=True, null=True)
class Meta:
ordering = ['order']
Some records have null as its "order" value.
If I get queryset, then the records that has null as order value are placed at beginning of result.
I just want to place such records at the end of queryset.
I've checked some articles for this purpose.
django order by isnull value?
Django : Order by position ignoring NULL
But I need it to be done using Django model's Meta class.
Is there any idea to do it via Meta class?
Regards
Philip