Designing the Model for Nested Objects for Python Mongoengine

I am new to ORM modeling in python.

Can someone help me to design a Class model in python mongoengine.

I want this structure to be inserted into the mongo database.

{
    "_id": ObjectId("5fedb7634600c11c2f3d38df"),
    "parent": {
        "_ref": {
            "$ref": "collection_a",
            "$id": ObjectId("5cddb7634600c11c2f343adf")
        },
        "status": "active"
    },
    "children": [
        {
            "_ref": {
                "$ref": "collection_a",
                "$id": ObjectId("5cedb7634600c11c2f343adf")
            },
            "status": "active"
        },
        {
            "_ref": {
                "$ref": "collection_a",
                "$id": ObjectId("5bedb7634600c11c2f343adf")
            },
            "status": "in-active"
        }
    ]
}

I have tried with

class ParentChildReference(EmbeddedDocument):
    ref = GenericReferenceField(choices='collection_a', dbref=False, required=True)
    status= StringField(required=True)

    def __str__(self):
        return 'ref={o.ref},status={o.status}'.format(o=self)

class ParentChildTree(Document):

    parent = EmbeddedDocumentField(ParentChildReference)
    children = EmbeddedDocumentListField(ParentChildReference)

    meta = {
        'auto_create_index': True,
        'collection': 'parent_dependency_tree'
    }

    def __str__(self):
        return 'parent ={o.parent},children ={o.children}'.format(o=self)

I am getting error like The source SON object needs to be of type 'dict' upon saving the document, Please suggest me here.

Thanks in advance

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