EmbeddedDocumentList in nested embededded document save() does not find the parent document

save() not working in an EmbeddedDocumentList in nested embedded document, as _instance is an EmbeddedDocument instead of Document

Reproducer:

class TestK2(EmbeddedDocument):
  values = EmbeddedDocumentListField(StringField, default=[])

class TestK1(EmbeddedDocument): 
  k2 = EmbeddedDocumentField(TestK2)

class Main(Document):
  part = EmbeddedDocumentField(TestK1)

def main(): 
  main = Part()
  main.part.k1.k2.values.save()  # AttributeError: TestK1 has no attribute 'save'

I would assume that nested EmbeddedDocuments should have _instance pointing to the Document instance and _parent pointing to the parent doc (embedded or actual document)

Start with the reproducer and the EmbeddedDocumentList save() entry point, then trace how _instance and _parent are assigned through the nested TestK1 and TestK2 embedded documents. Done means calling save() on the nested values list finds the Main Document rather than treating TestK1 as the document, without the reported AttributeError.