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Introduction to MongoDB in Python

Donny Winston

Instructor

Field paths

expression object ? {field1: <expression1>, ...}

db.laureates.aggregate([
    {"$project": {"prizes.share": 1}}
]).next()

{'_id': ObjectId('5bd3a610053b1704219e19d4'), 
                 'prizes': [{'share': '1'}]}

expression: 1

db.laureates.aggregate([
    {"$project": {"n_prizes": {"$size": "$prizes"}}}
]).next()

{'_id': ObjectId('5bd3a610053b1704219e19d4'), 
                  'n_prizes': 1}

expression: {"$size": "$prizes"}
field path: $prizes

Operator expressions

db.laureates.aggregate([
    {"$project": {"n_prizes": {"$size": "$prizes"}}}
]).next()

{'_id': ObjectId('5bd3a610053b1704219e19d4'), 'n_prizes': 1}

operator expression: {"$size": "$prizes"}
field path: $prizes

db.laureates.aggregate([
    {"$project": {"n_prizes": {"$size": ["$prizes"]}}}
]).next()

{'_id': ObjectId('5bd3a610053b1704219e19d4'), 'n_prizes': 1}

One more example: a multi-parameter operator

db.laureates.aggregate([
    {"$project": {"solo_winner": {"$in": ["1", "$prizes.share"]}}}
]).next()

{'_id': ObjectId('5bd3a610053b1704219e19d4'), 'solo_winner': True}

Implementing .distinct()

list_1 = db.laureates.distinct("bornCountry")

list_2 = [doc["_id"] for doc in db.laureates.aggregate([
    {"$group": {"_id": "$bornCountry"}}
])]
set(list_2) - {None} == set(list_1)

True

$groupmust map _id, which must be unique (like any Mongo document)
No $matchbefore $group
- All distinct "bornCountry" values captured
- including "no value" (None)

How many prizes have been awarded in total?

list(db.laureates.aggregate([
    {"$project": {"n_prizes": {"$size": "$prizes"}}},
    {"$group": {"_id": None, "n_prizes_total": {"$sum": "$n_prizes"}}}
]))

[{'_id': None, 'n_prizes_total': 941}]

{"_id": None"}? one document out.
$sumoperator acts as accumulator in $groupstage

Let's practice!

Introduction to MongoDB in Python