如何从具有选定字段的PyMongo中获取不同的值？

from pymongo import MongoClient

# 连接到MongoDB数据库
client = MongoClient('mongodb://localhost:27017/')
db = client['mydatabase']
collection = db['mycollection']

# 使用聚合框架获取不同的值
pipeline = [
    {
        '$group': {
            '_id': None,
            'field_values': {'$addToSet': '$field'}
        }
    },
    {
        '$project': {
            '_id': 0,
            'field_values': 1
        }
    }
]

result = collection.aggregate(pipeline)

# 打印结果
for doc in result:
    print(doc['field_values'])