Unable to save existing model instance created via _from_json method

Question

Method from_json of mongoengine ModelClass used for:

Converts json data to an unsaved document instance

If i send to a server json with some changed fields and trying to convert in to unsaved document instance, they dont track a _changed_fields in model eg _changed_fields is always empty.

data = self.request.data  # json representation of object
model_instance = SomeModel.from_json()
print(model_instance._changed_fields)  # get empty list

As i know .save method of ModelInstance only save a changed fields and check it in _changed_fields property. So if this property is always empty - unable to save model instance created via from_json method.

Question is - how can i save model instance created from json?

Answer 1

Can't you just do this?

data = self.request.data
model_instance = SomeModel(**data)
print(model_instance._changed_fields)  # should not be empty
model_instance.save()  # should work

Explanations:

In python, if a function or a class takes arguments (args) and named arguments (kwargs), one can populate them from respectively a list/tuple (my_list) and a dictionary (my_dict).

Requirements:

• The list/tuple must have as many elements as the number of positional arguments
• The dictionary can have as many keys/values we want, but only the ones matching the named arguments will be matched .

Example:

def foo(a, b, c=None, d=1):
    pass

class Foo(object):
    def __init__(self, a, b, c=None, d=1):
        pass

my_list = [1, 2]
my_dict = {'c': 3, 'd': 4}

you can do:

test = foo(*my_list, **my_dict)

or

obj = Foo(*my_list, **my_dict)

I hope it helps.