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

@@ -230,3 +230,26 @@ pipeline = [
 print(list(db.laureates.aggregate(pipeline)))
 ```
       [{'awardedElsewhere': 478}]
+      
+## 🦍 Refinement: filter out "unaffiliated" people
+- [x] Construct a stage added_stage that filters for laureate "prizes.affiliations.country" values that are non-empty, that is, are $in a list of the distinct values that the field takes in the collection.
+- [x] Insert this stage into the pipeline so that it filters out single prizes (not arrays) and precedes any test for membership in an array of countries. Recall that the first parameter to <list>.insert is the (zero-based) index for insertion.
+```py
+pipeline = [
+    {"$match": {"gender": {"$ne": "org"}}},
+    {"$project": {"bornCountry": 1, "prizes.affiliations.country": 1}},
+    {"$unwind": "$prizes"},
+    {"$addFields": {"bornCountryInAffiliations": {"$in": ["$bornCountry", "$prizes.affiliations.country"]}}},
+    {"$match": {"bornCountryInAffiliations": False}},
+    {"$count": "awardedElsewhere"},
+]
+
+# Construct the additional filter stage
+added_stage = {"$match": {"prizes.affiliations.country": {
+    "$in": db.laureates.distinct("prizes.affiliations.country")}}}
+
+# Insert this stage into the pipeline
+pipeline.insert(3, added_stage)
+print(list(db.laureates.aggregate(pipeline)))
+```
+      [{'awardedElsewhere': 252}]