讲一个简单的例子，python连接mongodb做跨表跨库查询的正确姿势

1.数据准备：

use order_db;
db.createCollection("orders");
db.orders.insertMany([{"_id": 1,"order_number": "ORD123","product": "Laptop","customer_id": 101},{"_id": 2,"order_number": "ORD124","product": "Smartphone","customer_id": 102},{"_id": 3,"order_number": "ORD125","product": "Tablet","customer_id": 103}
])use customer_db;
db.createCollection("customers");
db.customers.insertMany([{"_id": 101,"name": "John Doe","email": "john@example.com","address": "123 Main St"},{"_id": 102,"name": "Jane Smith","email": "jane@example.com","address": "456 Oak Ave"},{"_id": 103,"name": "Bob Johnson","email": "bob@example.com","address": "789 Pine Blvd"}
]);

2.跨集合查询

from pymongo import MongoClient## Joint Table Quety
# 连接到 MongoDB 数据库
client = MongoClient("mongodb://admin:admin@localhost:27017/")# 选择数据库和集合
db_orders = client.order_db.orders
db_customers = client.customer_db.customers# 执行跨表查询
pipeline = [{"$lookup": {"from": "customers","localField": "customer_id","foreignField": "_id","as": "customer_info"}},{"$unwind": "$customer_info"},{"$project": {"_id": 1,"order_number": 1,"product": 1,"customer_info.name": 1,"customer_info.email": 1}}
]result = list(db_orders.aggregate(pipeline))# 打印结果
for order in result:print(order)

分析：
经过代码测试会发现，pipeline走到lookup结束，customer_info为空，lookup是作用于单个数据库下的不同集合之间的联合查询，但不支持跨库，而网络上充斥着所谓支持跨库查询的案例。。。
因此，将collection放于同一个db下，发现结果符合预期。

3.跨库查询

应该怎么做？

思考：想象我们做的业务，通常都是模块化的，之间都是通过服务/应用层接口调用来实现的，其底层正对应着不同的数据库。比如常见的订单系统和用户系统，因为集中式管理(单个数据库)容易造成性能瓶颈，会按业务进行合理拆分，也更容易复用和拓展。

所以，所谓的跨库查询，实际上就跟业务之间的通信是类似的，这里并不是单库下的主外键查询问题，而是实际场景中多库下多个服务之间的数据互通与一致性查询问题，一般处理手段是将一些联合查询问题放到业务层解决，当然，针对做不同数据库的相同表做同步复制也是可以的，不过显然这与业务拆分的初衷相违背了。

以下是简单的sample,样例数据保持不变：

from pymongo import MongoClient## Cross Database Query# 连接到 MongoDB 数据库
client_db1 = MongoClient("mongodb://admin:admin@localhost:27017/")
client_db2 = MongoClient("mongodb://admin:admin@localhost:27017/")# 选择数据库和集合
customer_db = client_db1.customer_db
order_db = client_db2.order_dbcustomers_collection = customer_db.customers
orders_collection = order_db.orders# 查询 orders 数据
orders_data = list(orders_collection.find())
# 查询 customers 数据
customers_data_dict = {customer["_id"]: customer for customer in customers_collection.find()}
# 手动关联数据
result = []
for order in orders_data:customer_id = order.get("customer_id")# 在 customers 数据中查找匹配的 customer_idmatching_customer = customers_data_dict.get(customer_id)if matching_customer:# 合并数据merged_data = {**order, "customer_info": matching_customer}result.append(merged_data)# 打印结果
for item in result:print(item)

结果符合预期：