I am trying to build a framework that automatically extends model classes with additional fields. Here is a short summary of what I am trying to do:
Given a model class
class Pizza(models.Model):
name = models.CharField(max_length=10)
price = models.DecimalField(max_digits=10, decimal_places=2)
I automatically want to generate a class with an additional field per class field yielding a class similar to the following:
class PizzaGenerated(models.Model):
name = models.CharField(max_length=10)
name_new = models.CharField(max_length=10)
price = models.DecimalField(max_digits=10, decimal_places=2)
price_new = models.DecimalField(max_digits=10, decimal_places=2)
as you can see, for each of Pizza's properties, an additional field with the _new suffix has been added.
I need my solution to work irregardless of the Model's structure. In particular, I am looking for a way that allows the replication of ForeignKey-Fields
The above example of extending the Pizza class is solvable with the following code:
class ResMetaclass(models.base.ModelBase):
def __new__(cls, name, bases, attrs):
fields = {
k: v for k, v in attrs.items() if not k.startswith('_') and isinstance(v, models.Field)
}
attrs_extended = {
**attrs,
**{fieldname + '_new': fieldtype.clone() for fieldname, fieldtype in fields.items()}
}
bases = (models.Model,)
clsobj = super().__new__(cls, name, bases, attrs_extended)
return clsobj
class EntityBase(models.Model, metaclass=ResMetaclass):
class Meta:
abstract = True
class Pizza(EntityBase):
name = models.CharField(max_length=10)
price = models.DecimalField(max_digits=10, decimal_places=2)
The Pizza class is extended successfully by the EntityMetaclass metaclass.
Unfortunately, the above code fails, when the model contains a ForeignKey-Field, yielding the following backtrace:
File "manage.py", line 17, in main
execute_from_command_line(sys.argv)
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
utility.execute()
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/core/management/__init__.py", line 357, in execute
django.setup()
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/__init__.py", line 24, in setup
apps.populate(settings.INSTALLED_APPS)
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/apps/registry.py", line 114, in populate
app_config.import_models()
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/apps/config.py", line 211, in import_models
self.models_module = import_module(models_module_name)
File "/usr/lib64/python3.7/importlib/__init__.py", line 127, in import_module
return _bootstrap._gcd_import(name[level:], package, level)
File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
File "<frozen importlib._bootstrap>", line 983, in _find_and_load
File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
File "<frozen importlib._bootstrap_external>", line 728, in exec_module
File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
File "/home/niklas/dev/pyflx/mig/models.py", line 20, in <module>
class Contact(EntityBase):
File "/home/niklas/dev/pyflx/pyflx/models.py", line 17, in __new__
**{fieldname + '_patch_value': fieldtype.clone() for fieldname, fieldtype in fields.items() if fieldname != 'id'},
File "/home/niklas/dev/pyflx/pyflx/models.py", line 17, in <dictcomp>
**{fieldname + '_patch_value': fieldtype.clone() for fieldname, fieldtype in fields.items() if fieldname != 'id'},
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/db/models/fields/__init__.py", line 492, in clone
name, path, args, kwargs = self.deconstruct()
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/db/models/fields/related.py", line 856, in deconstruct
name, path, args, kwargs = super().deconstruct()
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/db/models/fields/related.py", line 583, in deconstruct
swappable_setting = self.swappable_setting
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/db/models/fields/related.py", line 374, in swappable_setting
return apps.get_swappable_settings_name(to_string)
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/apps/registry.py", line 288, in get_swappable_settings_name
for model in self.get_models(include_swapped=True):
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/apps/registry.py", line 178, in get_models
self.check_models_ready()
File "/home/niklas/dev/pyflx/venv/lib64/python3.7/site-packages/django/apps/registry.py", line 140, in check_models_ready
raise AppRegistryNotReady("Models aren't loaded yet.")
django.core.exceptions.AppRegistryNotReady: Models aren't loaded yet.
Is there a way to get around this?
Tricky!
The problem there is that when you try to .clone the field in the models, they have not being fully initialized, and thus, Django mechanisms of finding a foreign-model by its string qualifeid name can't be used. The problem is that the code does not check if the foreignmodel is passed as an class reference, instead of a string.
The only way to workaround this seems to be monkey-patching these checks when the classes are being created.
While at that, when clonning a foreign-object field, the related_field - a backreference created automatically by django ORM so one can get from the "pointed-at" object to the "holder" object have to be passed explicitly to the new, cloned, field. Else it would point to the original field instead.
This requires a bit more of monkeypatching, to insert an explicit "related_name" parameter in the inner workings of the .clone call.
Those 2 things being acomplished, it seems to work. Here is is the code I used, based on yours:
from django.db import models
from django.db.models.fields import related
from unittest.mock import patch
class ResMetaclass(models.base.ModelBase):
def __new__(cls, name, bases, attrs):
fields = {
k: v for k, v in attrs.items() if not k.startswith('_') and isinstance(v, models.Field)
}
new_fields = {}
for field_name, field in fields.items():
new_field_name = field_name + "_new"
if not isinstance(field, related.RelatedField):
new_fields[new_field_name] = field.clone()
else:
real_deconstruct = field.deconstruct
def _deconstruct():
name, path, args, kwargs = real_deconstruct()
kwargs["related_name"] = new_field_name
return name, path, args, kwargs
with patch("django.apps.registry.apps.check_models_ready", lambda: True):
field.deconstruct = _deconstruct
# Assume foregnKeys are always within the same file, and
# disable model-ready checking:
new_fields[new_field_name] = field.clone()
del field.deconstruct
attrs_extended = {
**attrs,
**new_fields
}
bases = (models.Model,)
clsobj = super().__new__(cls, name, bases, attrs_extended)
return clsobj
class EntityBase(models.Model, metaclass=ResMetaclass):
class Meta:
abstract = True
class Pizza(EntityBase):
name = models.CharField(max_length=10)
price = models.DecimalField(max_digits=10, decimal_places=2)
class MenuEntry(EntityBase):
entry_number = models.IntegerField()
pizza = models.ForeignKey("Pizza", on_delete="cascade")
And the resulting fields on the MenuEntry class:
In [1]: from test1.models import Pizza, MenuEntry
In [2]: MenuEntry._meta.fields
Out[2]:
(<django.db.models.fields.AutoField: id>,
<django.db.models.fields.IntegerField: entry_number>,
<django.db.models.fields.related.ForeignKey: pizza>,
<django.db.models.fields.IntegerField: entry_number_new>,
<django.db.models.fields.related.ForeignKey: pizza_new>)