Save reference field mongoengine

Question

Simplified, I have 2 Document objects: Resource and Cable

class Cable(db.Document):
    _id = db.ObjectIdField()
    socket = db.EmbeddedDocumentField(Socket)

class Resource(db.Document):
    _id = db.StringField()
    cable = db.ReferenceField('Cable')

Both the documents are already in the db, but the cable field in the resource is set to null .

@resources.route('/<r_id>/add_cabling', methods=['GET'])
def set_connector(r_id):
    r = Resource.objects(id=r_id).get()
    c = Cable.objects().first()
    r.cable=c
    r.save()
    return jsonify(r)

So I am passing the instance of the document Cable to the instance of Resource and then save() . The error that I get is the following:

ValidationError: ValidationError (Resource:res01) (A ReferenceField only accepts DBRef or documents: ['cable'])

I don't understand because actually I am passing the document itself

Answer 1

Tried with this workaround passing the DBRef of the document and it works.

@resources.route('/<r_id>/add_cabling', methods=['GET'])
def set_connector(r_id):
    r = Resource.objects(id=r_id).get()
    c = Cable.objects().first()
    c.save() #revalidate here
    r.cable=c.to_dbref()
    r.save()
    return jsonify(r)

It is necessary to perform the save() operation again on the queried object to get the DBRef otherwise it will throw this error:

OperationError: Only saved documents can have a valid dbref