add support for math database functions

.github/workflows/test-python.yml

@@ -74,6 +74,7 @@ jobs:
           bulk_create
           dates
           datetimes
+          db_functions.math
           empty
           defer
           defer_regress

django_mongodb/features.py

@@ -54,12 +54,20 @@ class DatabaseFeatures(BaseDatabaseFeatures):
         "lookup.tests.LookupTests.test_pattern_lookups_with_substr",
         # Querying ObjectID with string doesn't work.
         "lookup.tests.LookupTests.test_lookup_int_as_str",
+        # MongoDB gives the wrong result of log(number, base) when base is a
+        # fractional Decimal: https://jira.mongodb.org/browse/SERVER-91223
+        "db_functions.math.test_log.LogTests.test_decimal",
+        # MongoDB gives ROUND(365, -1)=360 instead of 370 like other databases.
+        "db_functions.math.test_round.RoundTests.test_integer_with_negative_precision",
     }
 
     django_test_skips = {
         "Insert expressions aren't supported.": {
             "bulk_create.tests.BulkCreateTests.test_bulk_insert_now",
             "bulk_create.tests.BulkCreateTests.test_bulk_insert_expressions",
+            # PI()
+            "db_functions.math.test_round.RoundTests.test_decimal_with_precision",
+            "db_functions.math.test_round.RoundTests.test_float_with_precision",
         },
         "Pattern lookups on UUIDField are not supported.": {
             "model_fields.test_uuid.TestQuerying.test_contains",
@@ -292,4 +300,28 @@ class DatabaseFeatures(BaseDatabaseFeatures):
             "timezones.tests.NewDatabaseTests.test_aware_datetime_in_local_timezone_with_microsecond",
             "timezones.tests.NewDatabaseTests.test_naive_datetime_with_microsecond",
         },
+        "Transform not supported.": {
+            "db_functions.math.test_abs.AbsTests.test_transform",
+            "db_functions.math.test_acos.ACosTests.test_transform",
+            "db_functions.math.test_asin.ASinTests.test_transform",
+            "db_functions.math.test_atan.ATanTests.test_transform",
+            "db_functions.math.test_ceil.CeilTests.test_transform",
+            "db_functions.math.test_cos.CosTests.test_transform",
+            "db_functions.math.test_cot.CotTests.test_transform",
+            "db_functions.math.test_degrees.DegreesTests.test_transform",
+            "db_functions.math.test_exp.ExpTests.test_transform",
+            "db_functions.math.test_floor.FloorTests.test_transform",
+            "db_functions.math.test_ln.LnTests.test_transform",
+            "db_functions.math.test_radians.RadiansTests.test_transform",
+            "db_functions.math.test_round.RoundTests.test_transform",
+            "db_functions.math.test_sin.SinTests.test_transform",
+            "db_functions.math.test_sqrt.SqrtTests.test_transform",
+            "db_functions.math.test_tan.TanTests.test_transform",
+        },
+        "MongoDB does not support Sign.": {
+            "db_functions.math.test_sign.SignTests",
+        },
+        "MongoDB can't annotate ($project) a function like PI().": {
+            "db_functions.math.test_pi.PiTests.test",
+        },
     }

django_mongodb/functions.py

@@ -1,8 +1,25 @@
 from django.db import NotSupportedError
+from django.db.models.expressions import Func
 from django.db.models.functions.datetime import Extract
+from django.db.models.functions.math import Ceil, Cot, Degrees, Log, Power, Radians, Random, Round
+from django.db.models.functions.text import Upper
 
 from .query_utils import process_lhs
 
+MONGO_OPERATORS = {
+    Ceil: "ceil",
+    Degrees: "radiansToDegrees",
+    Power: "pow",
+    Radians: "degreesToRadians",
+    Random: "rand",
+    Upper: "toUpper",
+}
+
+
+def cot(self, compiler, connection):
+    lhs_mql = process_lhs(self, compiler, connection)
+    return {"$divide": [1, {"$tan": lhs_mql}]}
+
 
 def extract(self, compiler, connection):
     lhs_mql = process_lhs(self, compiler, connection)
@@ -17,5 +34,28 @@ def extract(self, compiler, connection):
     return {operator: lhs_mql}
 
 
+def func(self, compiler, connection):
+    lhs_mql = process_lhs(self, compiler, connection)
+    operator = MONGO_OPERATORS.get(self.__class__, self.function.lower())
+    return {f"${operator}": lhs_mql}
+
+
+def log(self, compiler, connection):
+    # This function is usually log(base, num) but on MongoDB it's log(num, base).
+    clone = self.copy()
+    clone.set_source_expressions(self.get_source_expressions()[::-1])
+    return func(clone, compiler, connection)
+
+
+def round_(self, compiler, connection):
+    # Round needs its own function because it's a special case that inherits
+    # from Transform but has two arguments.
+    return {"$round": [expr.as_mql(compiler, connection) for expr in self.get_source_expressions()]}
+
+
 def register_functions():
+    Cot.as_mql_agg = cot
     Extract.as_mql = extract
+    Func.as_mql_agg = func
+    Log.as_mql_agg = log
+    Round.as_mql_agg = round_

django_mongodb/operations.py

@@ -23,6 +23,8 @@ def adapt_datetimefield_value(self, value):
 
     def adapt_decimalfield_value(self, value, max_digits=None, decimal_places=None):
         """Store DecimalField as Decimal128."""
+        if value is None:
+            return None
         return Decimal128(value)
 
     def adapt_timefield_value(self, value):

django_mongodb/query_utils.py

@@ -6,6 +6,10 @@ def is_direct_value(node):
 
 
 def process_lhs(node, compiler, connection, bare_column_ref=False):
+    if not hasattr(node, "lhs"):
+        # node is a Func or Expression, possibly with multiple source expressions.
+        return [expr.as_mql(compiler, connection) for expr in node.get_source_expressions()]
+    # node is a Transform with just one source expression, aliased as "lhs".
     if is_direct_value(node.lhs):
         return node
     mql = node.lhs.as_mql(compiler, connection)