add TruncTime support

Author: timgraham

README.md

@@ -204,7 +204,6 @@ Congratulations, your project is ready to go!
     - `SHA1`, `SHA224`, `SHA256`, `SHA384`, `SHA512`
     - `Sign`
     - `TruncDate`
-    - `TruncTime`
 
 - The `tzinfo` parameter of the `Trunc` database functions doesn't work
   properly because MongoDB converts the result back to UTC.

django_mongodb/features.py

@@ -503,11 +503,6 @@ def django_test_expected_failures(self):
             "model_fields.test_datetimefield.DateTimeFieldTests.test_lookup_date_without_use_tz",
             "timezones.tests.NewDatabaseTests.test_query_convert_timezones",
         },
-        "TruncTime database function not supported.": {
-            "db_functions.datetime.test_extract_trunc.DateFunctionTests.test_trunc_time_comparison",
-            "db_functions.datetime.test_extract_trunc.DateFunctionTests.test_trunc_time_func",
-            "db_functions.datetime.test_extract_trunc.DateFunctionTests.test_trunc_time_none",
-        },
         "MongoDB can't annotate ($project) a function like PI().": {
             "aggregation.tests.AggregateTestCase.test_aggregation_default_using_decimal_from_database",
             "db_functions.math.test_pi.PiTests.test",

django_mongodb/functions.py

@@ -16,6 +16,7 @@
     Now,
     TruncBase,
     TruncDate,
+    TruncTime,
 )
 from django.db.models.functions.math import Ceil, Cot, Degrees, Log, Power, Radians, Random, Round
 from django.db.models.functions.text import (
@@ -199,6 +200,23 @@ def trunc_date(self, compiler, connection):
     return {"$toDate": lhs_mql}
 
 
+def trunc_time(self, compiler, connection):
+    lhs_mql = process_lhs(self, compiler, connection)
+    return {
+        "$dateFromString": {
+            "dateString": {
+                "$concat": [
+                    # Times are stored with datetime.min.date(), so by
+                    # replacing any existing date component with that, the
+                    # result of TruncTime can be compared to TimeField.
+                    "0001-01-01T",
+                    {"$dateToString": {"format": "%H:%M:%S.%L", "date": lhs_mql}},
+                ]
+            }
+        }
+    }
+
+
 def register_functions():
     Cast.as_mql = cast
     Concat.as_mql = concat
@@ -221,4 +239,5 @@ def register_functions():
     Trim.as_mql = trim("trim")
     TruncBase.as_mql = trunc
     TruncDate.as_mql = trunc_date
+    TruncTime.as_mql = trunc_time
     Upper.as_mql = preserve_null("toUpper")