Limits and Skips with Sorts, Oh My!

Introduction to MongoDB in Python

Donny Winston

Instructor

Limiting our exploration

for doc in db.prizes.find({}, ["laureates.share"]):
    share_is_three = [laureate["share"] == "3"
                      for laureate in doc["laureates"]]
    assert all(share_is_three) or not any(share_is_three)

for doc in db.prizes.find({"laureates.share": "3"}):
    print("{year} {category}".format(**doc))

2017 chemistry
2017 medicine
2016 chemistry
2015 chemistry
2014 physics
2014 chemistry
2013 chemistry
...

for doc in db.prizes.find({"laureates.share": "3"}, limit=3):
    print("{year} {category}".format(**doc))

2017 chemistry
2017 medicine
2016 chemistry

Skips and paging through results

for doc in db.prizes.find({"laureates.share": "3"}, limit=3):
    print("{year} {category}".format(**doc))

2017 chemistry
2017 medicine
2016 chemistry

for doc in db.prizes.find({"laureates.share": "3"}, skip=3, limit=3):
    print("{year} {category}".format(**doc))

2015 chemistry
2014 physics
2014 chemistry

for doc in db.prizes.find({"laureates.share": "3"}, skip=6, limit=3):
    print("{year} {category}".format(**doc))

2013 chemistry
2013 medicine
2013 economics

Using cursor methods for {sort, skip, limit}

for doc in db.prizes.find({"laureates.share": "3"}).limit(3):
    print("{year} {category}".format(**doc))

2017 chemistry
2017 medicine
2016 chemistry

for doc in (db.prizes.find({"laureates.share": "3"}).skip(3).limit(3)):
    print("{year} {category}".format(**doc))

2015 chemistry
2014 physics
2014 chemistry

for doc in (db.prizes.find({"laureates.share": "3"})
            .sort([("year", 1)])
            .skip(3)
            .limit(3)):
    print("{year} {category}".format(**doc))

1954 medicine
1956 physics
1956 medicine

Simpler sorts of sort

cursor1 = (db.prizes.find({"laureates.share": "3"}).skip(3).limit(3)
          .sort([("year", 1)]))

cursor2 = (db.prizes.find({"laureates.share": "3"}).skip(3).limit(3)
          .sort("year", 1))

cursor3 = (db.prizes.find({"laureates.share": "3"}).skip(3).limit(3)
          .sort("year"))

docs = list(cursor1)
assert docs == list(cursor2) == list(cursor3)
for doc in docs:
    print("{year} {category}".format(**doc))

1954 medicine
1956 physics
1956 medicine

doc = db.prizes.find_one({"laureates.share": "3"},
                         skip=3, sort=[("year", 1)])
print("{year} {category}".format(**doc))

1954 medicine

Limit or Skip Practice? Exactly.

Introduction to MongoDB in Python