INTPYTHON-973 Add support for the Right database function

Author: timgraham

django_mongodb_backend/features.py

@@ -265,7 +265,6 @@ def django_test_expected_failures(self):
             "db_functions.text.test_pad.PadTests",
             "db_functions.text.test_repeat.RepeatTests",
             "db_functions.text.test_reverse.ReverseTests",
-            "db_functions.text.test_right.RightTests",
             "db_functions.text.test_sha1.SHA1Tests",
             "db_functions.text.test_sha224.SHA224Tests",
             "db_functions.text.test_sha256.SHA256Tests",

django_mongodb_backend/functions.py

@@ -33,6 +33,7 @@
     Lower,
     LTrim,
     Replace,
+    Right,
     RTrim,
     StrIndex,
     Substr,
@@ -118,6 +119,23 @@ def left(self, compiler, connection):
     return self.get_substr().as_mql(compiler, connection, as_expr=True)
 
 
+def right(self, compiler, connection):
+    expression, length = (
+        expr.as_mql(compiler, connection, as_expr=True) for expr in self.get_source_expressions()
+    )
+    # Calculate substring's start value by subtracting the requested length
+    # from the total length of the string.
+    start = {"$max": [{"$subtract": [{"$strLenCP": expression}, length]}, 0]}
+    return {
+        "$cond": {
+            "if": {"$eq": [length, None]},
+            # $substrCP returns "" for null length; return null instead.
+            "then": None,
+            "else": {"$substrCP": [expression, start, length]},
+        }
+    }
+
+
 def length(self, compiler, connection):
     # Check for null first since $strLenCP only accepts strings.
     lhs_mql = process_lhs(self, compiler, connection, as_expr=True)
@@ -296,6 +314,7 @@ def register_functions():
     Now.as_mql_expr = now
     NullIf.as_mql_expr = null_if
     Replace.as_mql_expr = replace
+    Right.as_mql_expr = right
     Round.as_mql_expr = round_
     RTrim.as_mql_expr = trim("rtrim")
     StrIndex.as_mql_expr = str_index

docs/releases/6.0.x.rst

@@ -10,6 +10,10 @@ Django MongoDB Backend 6.0.x
 New features
 ------------
 
+- Added support for the following database functions:
+
+  -  :class:`~django.db.models.functions.Right`
+
 - Added support for Django's warning when performing import-time queries.
 
 Bug fixes

docs/topics/known-issues.rst

@@ -83,7 +83,6 @@ Database functions
   - :class:`~django.db.models.functions.Ord`
   - :class:`~django.db.models.functions.Repeat`
   - :class:`~django.db.models.functions.Reverse`
-  - :class:`~django.db.models.functions.Right`
   - :class:`~django.db.models.functions.SHA1`,
     :class:`~django.db.models.functions.SHA224`,
     :class:`~django.db.models.functions.SHA256`,