Problem update dictionaries with a high degree of nesting using DynamicDocument

I try to repeat this:

In [1]: from mongoengine import *

In [2]: class TestData(DynamicDocument): pass

In [3]: td = TestData()

In [4]: td.mydata = [{'name':'bob','sdata':[]}]

In [5]: td.save()
Out[5]: <TestData: TestData object>

At this example we created new dynamic document.
Next we want to get this document and append some data to 'sdata' key in dictionary.

In [6]: td2 = TestData.objects.first()

In [7]: td2.mydata
Out[7]: [{'name': 'bob', 'sdata': []}]

In [8]: td2.mydata[0]['sdata'].append({'office':'current'})

In [9]: td2.mydata
Out[9]: [{'name': 'bob', 'sdata': [{'office': 'current'}]}]

In [10]: td2.save()
Out[10]: <TestData: TestData object>

Let's get this document again and check our changes

In [11]: td3 = TestData.objects.first()

In [12]: td3.mydata
Out[12]: [{'name': 'bob', 'sdata': []}]

As you can see mongoengine not saved new values in 'sdata' key

I found a solution how to get around this problem

In [13]: from copy import copy

In [14]: td4 = TestData.objects.first()

In [15]: ddd = copy(td4.mydata)

In [16]: ddd
Out[16]: [{'name': 'bob', 'sdata': []}]

In [17]: ddd[0]['sdata'].append({'office':'current'})

In [18]: ddd
Out[18]: [{'name': 'bob', 'sdata': [{'office': 'current'}]}]

In [19]: td4.mydata = copy(ddd)

In [20]: td4.save()
Out[20]: <TestData: TestData object>

In [21]: td5 = TestData.objects.first()

In [22]: td5.mydata
Out[22]: [{'name': 'bob', 'sdata': [{'office': 'current'}]}]

But this is not the right way.

Reproduce the nested-list update shown with DynamicDocument and MongoDB, then trace how the in-place append is tracked between loading and save. Done means appending to td2.mydata[0]['sdata'] persists after reloading the document, with a regression test covering the example.