No error when using ReferenceField to a document that doesn't exists

[Shlomi Lanton]

I'm trying to create a One-To-Many relationship between 2 document (classes), but I saw that I can create a new document with reference to another document that doesn't exist yet in the DB. 

Why is that? Shouldn't mongo enforce this relationship by making sure that the connected document exists before creating the new document? is there a flag that I should use to get this functionality?

Code example:

I have the following 2 classes:

class PayMethods(BaseDocument):

    name = StringField(required=True, max_length=200, unique=True)

class Expense(BaseDocument):

    amount = IntField()

    descreption = StringField(required=True, max_length=200)

    pay_method = ReferenceField(PayMethods, required=True)

In the DB I have the following `PayMethod` documents and no `Expense` documents:

{

    "_id" : ObjectId("5b5c8a2b2c88848042426dfd"),

    "name" : "MasterCard"

}

Now I create a new document:

Expense(amount=1, descreption='Test', pay_method=ObjectId("4c4c8a2b2c88848042426dfd"))

I expected to get an error message at that point, but it doesn't happen and I successfully create this document.

I'm Using:

Python 3.6
flask

mongoengine

flask-mongoengine