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

@@ -125,3 +125,15 @@ pipeline = [
 for doc in db.prizes.aggregate(pipeline):
     print("{year}: {missing}".format(year=doc["_id"],missing=", ".join(sorted(doc["missing"]))))
 ```
+## 🦍 Embedding aggregation expressions
+The `$expr` operator allows embedding of aggregation expressions in a normal query (or in a $match stage). Which of the following expressions counts the number of laureate documents with string-valued bornCountries when passed to `db.laureates.count_documents`?
+
+You can assume (and check!) that the following is true:
+
+      assert all(isinstance(v, str) for v in db.laureates.distinct("bornCountry"))
+Possible Answers
+- [ ] {"bornCountry": {"$in": db.laureates.distinct("bornCountry")}}
+- [ ] {"$expr": {"$in": ["$bornCountry", db.laureates.distinct("bornCountry")]}}
+- [ ] {"$expr": {"$eq": [{"$type": "$bornCountry"}, "string"]}}
+- [ ] {"bornCountry": {"$type": "string"}}
+- [ ] All of the above