当我提供findOne函数时，我得到了下面的错误。它显示为NULL，无法找到解决方案，因为有标题的记录。
db.links.findOne({title: "MongoDB TUtor"});
{
"_id" : ObjectId("54f3ea83663952fddd7aaa54"),
"title" : "MongoDB TUtor",
"url" : "mongo.com",
"comment" : "document-oriented db"
}
> db.links.findOne({title: "true"});
null
> db.links.findOne({title: 1});
null
-
链接集合中的记录
db.links.find().forEach(printjson)
{
"_id" : ObjectId("54f3ea83663952fddd7aaa54"),
"title" : "MongoDB TUtor",
"url" : "mongo.com",
"comment" : "document-oriented db"
}
{
"_id" : ObjectId("54f3ebe7663952fddd7aaa55"),
"title" : "Mongo DB handson",
"url" : "https://university.mongo.com",
"comment" : [
"NoSQL",
"Document-based"
],
"filed_on" : ISODate("2015-03-02T04:47:02.131Z"),
"meta" : {
"browser" : [
"Google Chrome",
"MAC Safari"
],
"version" : "13.4.3.7"
}
}
{
"_id" : ObjectId("54f3f77e663952fddd7aaa5a"),
"title" : "MongoDB_database",
"url" : "mongo.com",
"comment" : "document-oriented db"
}
{
"_id" : ObjectId("54f3f7b7663952fddd7aaa5c"),
"title" : "database",
"url" : "mongo.com",
"comment" : "document-oriented db"
}
{ "_id" : ObjectId("54f3fa5f663952fddd7aaa5d"), "title" : "Rocky" }
{
"_id" : ObjectId("54f3facf663952fddd7aaa5e"),
"title" : "Arnold",
"userId" : ObjectId("54f3fa5f663952fddd7aaa5d")
}
> db.links.find().pretty()
{
"_id" : ObjectId("54f3ea83663952fddd7aaa54"),
"title" : "MongoDB TUtor",
"url" : "mongo.com",
"comment" : "document-oriented db"
}
{
"_id" : ObjectId("54f3ebe7663952fddd7aaa55"),
"title" : "Mongo DB handson",
"url" : "https://university.mongo.com",
"comment" : [
"NoSQL",
"Document-based"
],
"filed_on" : ISODate("2015-03-02T04:47:02.131Z"),
"meta" : {
"browser" : [
"Google Chrome",
"MAC Safari"
],
"version" : "13.4.3.7"
}
}
{
"_id" : ObjectId("54f3f77e663952fddd7aaa5a"),
"title" : "MongoDB_database",
"url" : "mongo.com",
"comment" : "document-oriented db"
}
{
"_id" : ObjectId("54f3f7b7663952fddd7aaa5c"),
"title" : "database",
"url" : "mongo.com",
"comment" : "document-oriented db"
}
{ "_id" : ObjectId("54f3fa5f663952fddd7aaa5d"), "title" : "Rocky" }
{
"_id" : ObjectId("54f3facf663952fddd7aaa5e"),
"title" : "Arnold",
"userId" : ObjectId("54f3fa5f663952fddd7aaa5d")
}
答案 0 :(得分：1)
这不是你怎么做的。您查询它的方式是查找具有"title": true或"title": 1的实际文档。当然，你的文件都不符合这个条件。
如果您想知道该字段是否“实际存在”，那么您可以使用$exists运算符来测试该状态：
db.links.findOne({ "title": { "$exists": true } })
或者在查找没有匹配字段的文档时，反之亦然。