move stddev/variance logic to separate function

Author: timgraham

django_mongodb/aggregates.py

@@ -7,14 +7,10 @@
 
 from .query_utils import process_lhs
 
-MONGO_AGGREGATIONS = {
-    Count: "sum",
-    StdDev: "stdDev",  # Samp or Pop suffix added in aggregate().
-    Variance: "stdDev",  # Likewise.
-}
+MONGO_AGGREGATIONS = {Count: "sum"}
 
 
-def aggregate(self, compiler, connection, **extra_context):  # noqa: ARG001
+def aggregate(self, compiler, connection, operator=None, **extra_context):  # noqa: ARG001
     if self.filter:
         node = self.copy()
         node.filter = None
@@ -24,12 +20,7 @@ def aggregate(self, compiler, connection, **extra_context):  # noqa: ARG001
     else:
         node = self
     lhs_mql = process_lhs(node, compiler, connection)
-    operator = MONGO_AGGREGATIONS.get(self.__class__, self.function.lower())
-    # Add suffixes to StdDev/Variance.
-    if self.function.endswith("_SAMP"):
-        operator += "Samp"
-    elif self.function.endswith("_POP"):
-        operator += "Pop"
+    operator = operator or MONGO_AGGREGATIONS.get(self.__class__, self.function.lower())
     return {f"${operator}": lhs_mql}
 
 
@@ -69,6 +60,16 @@ def count(self, compiler, connection, resolve_inner_expression=False, **extra_co
     return {"$add": [{"$size": lhs_mql}, exits_null]}
 
 
+def stddev_variance(self, compiler, connection, **extra_context):
+    if self.function.endswith("_SAMP"):
+        operator = "stdDevSamp"
+    elif self.function.endswith("_POP"):
+        operator = "stdDevPop"
+    return aggregate(self, compiler, connection, operator=operator, **extra_context)
+
+
 def register_aggregates():
     Aggregate.as_mql = aggregate
     Count.as_mql = count
+    StdDev.as_mql = stddev_variance
+    Variance.as_mql = stddev_variance