MongoDB - 聚合

# 事例数据
{
    "_id": ObjectId("5e5e05ef88f52a1c16bbff0f"),
    "url": "https://www.baidu.com/",
    "web_name": "百度",
    "click_num": 605,
    "year": 2019
}

# 输出文档中除了web_name和click_num外，还包含默认的 _id 域
db.test_study.aggregate([
    {
        $project: {
            web_name: "$web_name",
            click_num: "$click_num"
        }
    }
])


# 设置 _id:0 输出文档中只包括 web_name 和 click_num，不包含 _id 域
db.test_study.aggregate([
    {
        $project: {
         _id:0,
            web_name: "$web_name",
            click_num: "$click_num"
        }
    }
])

# 事例数据{
    "_id": 1,
    "title": "789",
    "author": {
        "last": "Li",
        "first": "Lucy",
        "country": "China"
    },
    "copies": 5,
    "lastModified": "2019-07-28"
}

需求：输出 title，author.country

db.test.aggregate([{
    $project: {
    _id:0,
        title: "$title",
        author_country: "$author.country"
    }
}])

db.test.aggregate([{
    $project: {
        "_id":0,
        "copies": 0,
        "author.country": 0
    }
}])注意：
"copies" 要加双引号