I'm trying to query the last element of a list in a Mongo document using MongoEngine. I can write a raw query to do this but I'm wondering if it's possible to do it without resorting to that.
So for example, while
X.objects(some_list__val='Test Value') would return any objects which have some list element such that val=='Test Value', I'd like for it to only return those objects where val=='Test Value' in the final element of the list.
You can use $arrayElemAt or (from MongoDB 5.0 and later) $last :
from pymongo import MongoClient
db = MongoClient(port=27019)['mydatabase']
col = db['mycollection']
col.delete_many(({}))
col.insert_many([{'record': [1, 2, 3]},
{'record': [2, 3, 4]},
{'record': [3, 4, 5]}])
item_to_find = 4
agg = col.aggregate([
{'$project': {'record': '$record', 'last': {'$arrayElemAt': ["$record", -1]}}},
{'$match': {'last': item_to_find}}
])
print(list(agg))
agg = col.aggregate([
{'$project': {'record': '$record', 'last': {'$last': "$record"}}},
{'$match': {'last': item_to_find}}
])
print(list(agg))
prints:
[{'_id': ObjectId('6537065c5c20464e8749e8e5'), 'record': [2, 3, 4], 'last': 4}]
[{'_id': ObjectId('6537065c5c20464e8749e8e5'), 'record': [2, 3, 4], 'last': 4}]