Order_by length of listfield in mongoengine - mongodb

Question

I wan't to run a query to get all Articles that have more than 6 com and then sort according length of com list,
for this i doing it:
ArticleModel.objects.filter(com__6__exists=True).order_by('-com.length')[:50]
suppose com is a ListField, but ordering not work, how can i fix it? thanks

Answer 1

Standard queries cannot do this as the "sort" needs to be done on a physical field present in the document. The best way to do this is to actually keep a count of your "list" as another field in the document. That also makes your query more efficient as well as that "counter" field can be indexed, so the basic query becomes ( Raw MongoDB sytax ) :
{ "comLength": { "$gt": 6 } }
If you cannot or do not want to change the document structure then the only way to otherwise sort on the length of your list is to $project it via .aggregate():
ArticleModel._get_collection().aggregate([
{ "$match": { "com.6": { "$exists": true } }},
{ "$project": {
"com": 1,
"otherField": 1,
"comLength": { "$size": "$com" }
}},
{ "$sort": { "comLength": -1 } }
])
And that considers that you have MongoDB 2.6 at least for the use of the $size aggregation operator. If you don't then you have to $unwind and $group in order to calculate the length of arrays:
ArticleModel._get_collection().aggregate([
{ "$match": { "com.6": { "$exists": true } }},
{ "$unwind": "$com" },
{ "$group": {
"_id": "$_id",
"otherField": { "$first": "$otherField" }
"com": { "$push": "$com" },
"comLength": { "$sum": 1 }
}},
{ "$sort": { "comLength": -1 } }
])
So if you are going to go down that route then take a good look at the documentation since you are possibly not used to the raw MongoDB syntax and have been using the query DSL that MongoEngine provdides.
Overall, only the aggregation providers in .aggregate() or .mapReduce() can actually "create a field" that is not present within the document. There is also not test for the "current" length that is available to standard projection or sorting of documents either.
Your best option to to add another field and keep it in sync with the actual array length. But failing that the above shows you the general approach.

Answer 2

If you're creating the database and you know such request will mostly be requested a lot it's recommended to add "com_length" field in A ArticleModel and make it automatically inserted on every save using save() method
add inside of your ArticleModel in models.py
def save(self, *args, **kwargs):
self.com_length = len(self.com)
return super(ArticleModel, self).save(*args, **kwargs)
then for requesting the asked question:
ArticleModel.objects.filter(com__6__exists=True).order_by('-com_length')[:50]