Find, MongoDB. A real estate database might have a price for each house. We want to find just properties that have a field within a range of values (like a certain price).

With special range queries, used with the find() method, we can find documents with fields in a certain range. The $lt and $gt terms are useful here.

A program. Here we have a real estate database with 3 houses in it. Each house has a different price. We use find() to get matching houses with prices less than, or greater than, a value.

from pymongo import MongoClient

client = MongoClient("mongodb://127.0.0.1:27017")
db = client.real_estate

# Reset.
db.houses.delete_many({})

# Insert houses.
db.houses.insert_many([
    {"address": "100 Main St.", "price": 45000},
    {"address": "110 Main St.", "price": 50000},
    {"address": "1400 Rich Ave.", "price": 200000},
])

# Find inexpensive houses.
print("FIND PRICE LT 60000")
cursor = db.houses.find({"price": {"$lt": 60000}})
for doc in cursor:
    print(doc)

# Find expensive houses.
print("FIND PRICE GT 60000")
cursor = db.houses.find({"price": {"$gt": 60000}})
for doc in cursor:
    print(doc)
FIND PRICE LT 60000
{'address': '100 Main St.',
 '_id': ObjectId('59f122302514972194e7e975'), 'price': 45000}
{'address': '110 Main St.',
 '_id': ObjectId('59f122302514972194e7e976'), 'price': 50000}
FIND PRICE GT 60000
{'address': '1400 Rich Ave.',
 '_id': ObjectId('59f122302514972194e7e977'), 'price': 200000}

Notes, comparison operators. For small groups of documents, getting all documents and then checking them in an if-statement in Python is possible.

A review. Find() can be used with ranges. A dictionary argument with a special term (lt or gt) enables a range query. For large databases, this is essential.