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Remove $facet in top level group stages #449

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Remove $facet in top level group stages #449

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1 change: 1 addition & 0 deletions django_mongodb_backend/aggregates.py
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Original file line number Diff line number Diff line change
Expand Up @@ -52,6 +52,7 @@ def count(self, compiler, connection, resolve_inner_expression=False):
# If distinct=True or resolve_inner_expression=False, sum the size of the
# set.
lhs_mql = process_lhs(self, compiler, connection, as_expr=True)
lhs_mql = {"$ifNull": [lhs_mql, []]}
# None shouldn't be counted, so subtract 1 if it's present.
exits_null = {"$cond": {"if": {"$in": [{"$literal": None}, lhs_mql]}, "then": -1, "else": 0}}
return {"$add": [{"$size": lhs_mql}, exits_null]}
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18 changes: 4 additions & 14 deletions django_mongodb_backend/compiler.py
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Expand Up @@ -235,20 +235,10 @@ def _build_aggregation_pipeline(self, ids, group):
pipeline = []
if not ids:
group["_id"] = None
pipeline.append({"$facet": {"group": [{"$group": group}]}})
pipeline.append(
{
"$addFields": {
key: {
"$getField": {
"input": {"$arrayElemAt": ["$group", 0]},
"field": key,
}
}
for key in group
}
}
)
pipeline.append({"$group": group})
# It may be a bug, $$NOW has to be called to be reachable in the rest of the pipeline.
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@timgraham timgraham Nov 20, 2025

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Please add link to the SERVER ticket and add an explanatory comment about the purpose of the $unionWith clause.

I think we can also mention it in the release notes as a performance improvement since the team said $facet prevents the use of optimizations.

pipeline.append({"$set": {"__now": "$$NOW"}})
pipeline.append({"$unionWith": {"pipeline": [{"$documents": [{}]}]}})
else:
group["_id"] = ids
pipeline.append({"$group": group})
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