Update 4. Aggregation Pipelines: Let the Server Do It For You.md

Author: ortizfram

Introduction to MongoDB in Python/4. Aggregation Pipelines: Let the Server Do It For You.md

@@ -67,3 +67,17 @@ for key, group in groupby(cursor, key=itemgetter("year")):
         print("{year}: {missing}".format(
             year=key, missing=", ".join(sorted(missing))))
 ```
+## 🦍 Field Paths and Sets
+Previously, we confirmed -- via a Python loop -- that for each prize, either all laureates have a 1/3 share, or none do. Now, let's do this via an aggregation (result should be an empty list):
+
+      list(db.prizes.aggregate([
+          {"$project": {"allThree": {"$setEquals": [____, ____]},
+                        "noneThree": {"$not": {"$setIsSubset": [____, ____]}}}},
+          {"$match": {"$nor": [{"allThree": True}, {"noneThree": True}]}}]))
+> Which values fill the blanks?
+
+Possible Answers
+- [ ] "$laureates.share", ["3"], ["3"], "$laureates.share"
+- [ ] "laureates.share", ["3"], ["3"], "laureates.share"
+- [ ] "laureates.share", {"3"}, {"3"}, "laureates.share"
+- [ ] "$laureates.share", {"3"}, {"3"}, "$laureates.share"