Projection

# Inclusion projection — return only specified fields

from pymongo import MongoClient

client = MongoClient("mongodb://localhost:27017/")
db     = client["dataplexa"]

# Return only name and email — _id included by default
print("Name and email only (with _id):")
users = db.users.find({}, {"name": 1, "email": 1})
for u in users:
    print(f"  {u}")

# Suppress _id — clean output for an API response
print("\nName and email — no _id:")
users = db.users.find({}, {"name": 1, "email": 1, "_id": 0})
for u in users:
    print(f"  {u}")

# Product listing — name, price, rating only
print("\nProduct listing fields:")
products = db.products.find(
    {"category": "Electronics"},
    {"name": 1, "price": 1, "rating": 1, "_id": 0}
)
for p in products:
    print(f"  {p}")

# Exclusion projection — hide specific fields

from pymongo import MongoClient

client = MongoClient("mongodb://localhost:27017/")
db     = client["dataplexa"]

# Hide tags and joined — return everything else
print("Users without tags and joined fields:")
users = db.users.find({}, {"tags": 0, "joined": 0})
for u in users:
    print(f"  {u}")

# Hide internal/sensitive fields in a hypothetical users collection
# that had a password_hash field — never send it to the client
print("\nProducts without stock field (stock is internal):")
products = db.products.find(
    {},
    {"stock": 0, "_id": 0}
)
for p in products:
    print(f"  {p}")

# Dot notation projection — include or exclude nested fields

from pymongo import MongoClient

client = MongoClient("mongodb://localhost:27017/")
db     = client["dataplexa"]

# Imagine users had a nested address sub-document — query just the city
# db.users.find({}, {"address.city": 1, "name": 1, "_id": 0})

# Orders — project only the total and status, not the full items array
print("Order summaries (id, status, total only):")
orders = db.orders.find(
    {},
    {"status": 1, "total": 1, "_id": 1}
)
for o in orders:
    print(f"  {o['_id']}  status: {o['status']:12}  total: ${o['total']:.2f}")

# Project a nested field from embedded documents
# Show user_id and first item's product_id per order
print("\nOrders — user and items array:")
orders = db.orders.find(
    {},
    {"user_id": 1, "items": 1, "_id": 0}
)
for o in orders:
    item_ids = [item["product_id"] for item in o["items"]]
    print(f"  user: {o['user_id']}  products: {item_ids}")

# $slice — return a portion of an array field

from pymongo import MongoClient

client = MongoClient("mongodb://localhost:27017/")
db     = client["dataplexa"]

# Return only the first item from each order's items array
print("First item only from each order:")
orders = db.orders.find(
    {},
    {"user_id": 1, "items": {"$slice": 1}, "_id": 0}
)
for o in orders:
    print(f"  user: {o['user_id']}  first item: {o['items']}")

# Return only the last 1 item from each order
print("\nLast item only from each order:")
orders = db.orders.find(
    {},
    {"user_id": 1, "items": {"$slice": -1}, "_id": 0}
)
for o in orders:
    print(f"  user: {o['user_id']}  last item: {o['items']}")

# $slice with skip and limit — [skip, limit]
# Skip 1 item, return up to 1 item from the array
print("\nSecond item only (skip 1, take 1):")
orders = db.orders.find(
    {"user_id": "u001"},
    {"items": {"$slice": [1, 1]}, "_id": 0}
)
for o in orders:
    print(f"  items: {o.get('items', [])}")

# $elemMatch as a projection operator — return first matching array element

from pymongo import MongoClient

client = MongoClient("mongodb://localhost:27017/")
db     = client["dataplexa"]

# From each order, return only the item with product_id "p001" if present
print("Orders — show only the p001 line item if it exists:")
orders = db.orders.find(
    {"items.product_id": "p001"},        # filter: orders containing p001
    {
        "user_id": 1,
        "items": {"$elemMatch": {"product_id": "p001"}},  # projection
        "_id": 1
    }
)
for o in orders:
    print(f"  {o['_id']}  user: {o['user_id']}  matched item: {o.get('items')}")

# Compare: without $elemMatch projection — entire items array returned
print("\nWithout $elemMatch projection — full items array:")
orders = db.orders.find(
    {"items.product_id": "p001"},
    {"user_id": 1, "items": 1, "_id": 0}
)
for o in orders:
    print(f"  user: {o['user_id']}  all items: {[i['product_id'] for i in o['items']]}")

# Covered query concept — answered entirely from an index

from pymongo import MongoClient

client = MongoClient("mongodb://localhost:27017/")
db     = client["dataplexa"]

# Create a compound index on category and price
db.products.create_index([("category", 1), ("price", 1)])

# This query is now a covered query:
# filter  → uses category (in the index)
# projection → only category and price (both in the index)
# _id excluded → avoids fetching the full document

covered_result = db.products.find(
    {"category": "Electronics"},
    {"category": 1, "price": 1, "_id": 0}   # only indexed fields
)

print("Covered query — Electronics name and price:")
for p in covered_result:
    print(f"  {p}")

print("\nKey rule: if filter + projection fields are all in one index")
print("MongoDB never reads the actual document from disk — pure index scan")