python - How do I limit number of elements in updated array using $push and $slice in MongoEngine -


i have document array of subdocuments.

these document , subdocument like:

class activity_comment(embeddeddocument):     author_name = stringfield()     text_excerpt = stringfield()     url = stringfield()   class author_activity(document):     author_id = stringfield(required=true,primary_key=true)     author_name = stringfield()     author_bio = stringfield()     latest_comments = listfield(embeddeddocumentfield(activity_comment))

there multiple server processes modify the document @ same time. want use update_one method upsert=true.

i want restrict number of subdocuments in array 5. seems possible in mongodb according these links:

http://docs.mongodb.org/manual/tutorial/limit-number-of-elements-in-updated-array/ http://docs.mongodb.org/manual/reference/operator/update/slice/

these links suggest should use $push insert subdocuments array , $slice limit array length desired value.

however, haven't been able figure out how using mongoengine. tried following code 

author_activity.objects(author_id="1").update_one(push__latest_comments=activity_comment,slice__latest_comments=5, upsert=true)

but threw following exception:

traceback (most recent call last): file "", line 1, in file "/newsoftheworld/local/lib/python2.7/site-packages/mongoengine/queryset/base.py", line 467, in update_one upsert=upsert, multi=false, write_concern=write_concern, **update) file "/newsoftheworld/local/lib/python2.7/site-packages/mongoengine/queryset/base.py", line 430, in update update = transform.update(queryset._document, **update) file "/newsoftheworld/local/lib/python2.7/site-packages/mongoengine/queryset/transform.py", line 181, in update raise invalidqueryerror(e) invalidqueryerror: cannot resolve field "slice"

what findandmodify returted new value? if reached limit - remove one.


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