# 只包含特定字段
{"$project": {"name": 1, "email": 1}}

# 排除特定字段
{"$project": {"password": 0, "ssn": 0}}

{"$project": {"user_name": "$name", "contact": "$email"}}

{"$project": {
    "total": {"$add": ["$price", "$tax"]},
    "discounted": {"$multiply": ["$price", 0.9]}
}}

from pymongo import MongoClient

client = MongoClient('mongodb://localhost:27017/')
db = client['testdb']
collection = db['users']

# 示例1：基本字段选择
pipeline = [
    {"$project": {
        "username": 1,
        "email": 1,
        "registration_date": 1,
        "_id": 0
    }}
]
results = collection.aggregate(pipeline)

# 示例2：计算字段
pipeline = [
    {"$project": {
        "full_name": {"$concat": ["$first_name", " ", "$last_name"]},
        "years_since_joined": {
            "$subtract": [
                {"$year": {"$dateFromString": {"dateString": "2023-01-01"}}},
                {"$year": "$join_date"}
            ]
        }
    }}
]

# 示例3：条件投影
pipeline = [
    {"$project": {
        "name": 1,
        "status": {
            "$cond": {
                "if": {"$gte": ["$score", 70]},
                "then": "PASS",
                "else": "FAIL"
            }
        }
    }}
]

{"$project": {
    "has_email": {"$ifNull": ["$email", False]}
}}

{"$project": {
    "first_item": {"$arrayElemAt": ["$items", 0]},
    "item_count": {"$size": "$items"}
}}

{"$project": {
    "year": {"$year": "$date"},
    "month": {"$month": "$date"},
    "day": {"$dayOfMonth": "$date"}
}}

{"$project": {
    "string_id": {"$toString": "$_id"},
    "numeric_value": {"$toInt": "$string_number"}
}}