From 8e7078dd3ebd0e024b2dd5f4a62cc224cafaee00 Mon Sep 17 00:00:00 2001
From: Matthew Evans <me388@cam.ac.uk>
Date: Fri, 6 Mar 2020 18:57:35 +0000
Subject: [PATCH] Use $nor when $or -> $not chain is found

---
 optimade/filtertransformers/mongo.py   |  3 ++
 tests/filtertransformers/test_mongo.py | 48 ++++++++++++--------------
 2 files changed, 25 insertions(+), 26 deletions(-)

diff --git a/optimade/filtertransformers/mongo.py b/optimade/filtertransformers/mongo.py
index fa379de161..510cd34c1e 100755
--- a/optimade/filtertransformers/mongo.py
+++ b/optimade/filtertransformers/mongo.py
@@ -92,6 +92,9 @@ def expression_phrase(self, arg):
             # without NOT
             return arg[0]
 
+        if list(arg[1].keys()) == ["$or"]:
+            return {"$nor": arg[1]["$or"]}
+
         # with NOT
         # TODO: This implementation probably fails in the case of `"(" expression ")"`
         return {prop: {"$not": expr} for prop, expr in arg[1].items()}
diff --git a/tests/filtertransformers/test_mongo.py b/tests/filtertransformers/test_mongo.py
index ad05d0b4cf..49de5b81dd 100644
--- a/tests/filtertransformers/test_mongo.py
+++ b/tests/filtertransformers/test_mongo.py
@@ -114,30 +114,28 @@ def test_operators(self):
         self.assertEqual(self.transform("NOT a<3"), {"a": {"$not": {"$lt": 3}}})
 
         # TODO: {'$not': {'$eq': 'Ti'}} can be simplified to {'$ne': 'Ti'}
-        # TODO: THIS IS NOT A VALID MONGO QUERY!!! We should switch to $nor, but
-        # that may break the defined optimade operator precendence
         self.assertEqual(
             self.transform(
-                'NOT ( chemical_formula_hill = "Al" AND chemical_formula_anonymous = "A" OR '
-                'chemical_formula_anonymous = "H2O" AND NOT chemical_formula_hill = "Ti" )'
+                "NOT ( "
+                'chemical_formula_hill = "Al" AND chemical_formula_anonymous = "A" OR '
+                'chemical_formula_anonymous = "H2O" AND NOT chemical_formula_hill = "Ti"'
+                ")"
             ),
             {
-                "$or": {
-                    "$not": [
-                        {
-                            "$and": [
-                                {"chemical_formula_hill": {"$eq": "Al"}},
-                                {"chemical_formula_anonymous": {"$eq": "A"}},
-                            ]
-                        },
-                        {
-                            "$and": [
-                                {"chemical_formula_anonymous": {"$eq": "H2O"}},
-                                {"chemical_formula_hill": {"$not": {"$eq": "Ti"}}},
-                            ]
-                        },
-                    ]
-                }
+                "$nor": [
+                    {
+                        "$and": [
+                            {"chemical_formula_hill": {"$eq": "Al"}},
+                            {"chemical_formula_anonymous": {"$eq": "A"}},
+                        ]
+                    },
+                    {
+                        "$and": [
+                            {"chemical_formula_anonymous": {"$eq": "H2O"}},
+                            {"chemical_formula_hill": {"$not": {"$eq": "Ti"}}},
+                        ]
+                    },
+                ]
             },
         )
 
@@ -166,12 +164,10 @@ def test_operators(self):
                         "$and": [
                             {"nelements": {"$gte": 10}},
                             {
-                                "$or": {
-                                    "$not": [
-                                        {"_exmpl_x": {"$ne": "Some string"}},
-                                        {"_exmpl_a": {"$not": {"$eq": 7}}},
-                                    ]
-                                }
+                                "$nor": [
+                                    {"_exmpl_x": {"$ne": "Some string"}},
+                                    {"_exmpl_a": {"$not": {"$eq": 7}}},
+                                ]
                             },
                         ]
                     },