On update the model is not validated

Hi

For the following document model:

class Inventory(Document):
    store_id = StringField(required=True)
    product_sku = StringField(unique_with=['store_id'], required=True)
    is_product_enabled = BooleanField(default=False)
    stock = IntField(required=True, min_value=0, default=0)
    created_time = DateTimeField(default=datetime.now)
    updated_time = DateTimeField(default=datetime.now)

When running the following query to decrease the stock:

inventory_obj = Inventory.objects.get(product_sku=product_sku, store_id=store_id)
inventory_obj.update(dec__stock=10)

Document before the update:

{
    "_id" : ObjectId("57273c29aafbb3d5bf7b99f9"),
    "store_id" : "1",
    "product_sku" : "1",
    "is_product_enabled" : true,
    "ready_stock" : 7,
    "stock" : 6,
    "created_time" : ISODate("2016-05-02T17:08:17.452Z"),
    "updated_time" : ISODate("2016-05-02T17:26:39.756Z")
}

Post the update

{
    "_id" : ObjectId("57273c29aafbb3d5bf7b99f9"),
    "store_id" : "1",
    "product_sku" : "1",
    "is_product_enabled" : true,
    "stock" : -4,
    "created_time" : ISODate("2016-05-02T17:08:17.452Z"),
    "updated_time" : ISODate("2016-05-02T18:27:58.592Z")
}

The stock becomes negative. Is this the desired behaviour?
Is there a work around for validating the min_value?

Start at the Inventory.objects.get(...).update(dec__stock=10) entry point and trace how min_value validation is handled during atomic updates. Look for existing update-validation tests or documentation; done means the expected behavior for negative stock and any supported workaround are established with a reproducible test or clear documentation.