如何在MongoDB中使用$ toString将值转换为字符串？
让我们看一个示例，以了解MongoDB中的$toString。为了理解上述概念，让我们用文档创建一个集合。使用文档创建集合的查询如下-
> db.objectidToStringDemo.insertOne({"UserName":"John"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c92b80036de59bd9de0639d")
}
> db.objectidToStringDemo.insertOne({"UserName":"Chris"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c92b80436de59bd9de0639e")
}
> db.objectidToStringDemo.insertOne({"UserName":"Larry"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c92b80936de59bd9de0639f")
}
> db.objectidToStringDemo.insertOne({"UserName":"Robert"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c92b81836de59bd9de063a0")
}在find()method的帮助下显示集合中的所有文档。查询如下-
> db.objectidToStringDemo.find().pretty();
以下是输出-
{ "_id" : ObjectId("5c92b80036de59bd9de0639d"), "UserName" : "John" }
{ "_id" : ObjectId("5c92b80436de59bd9de0639e"), "UserName" : "Chris" }
{ "_id" : ObjectId("5c92b80936de59bd9de0639f"), "UserName" : "Larry" }
{ "_id" : ObjectId("5c92b81836de59bd9de063a0"), "UserName" : "Robert" }这是在MongoDB聚合中将ObjectId转换为字符串值的查询。查询如下-
> db.objectidToStringDemo.aggregate([
... {
... $project: {
... _id: {
... $toString: "$_id"
... }
... }
... }
... ]
... );以下是输出-
{ "_id" : "5c92b80036de59bd9de0639d" }
{ "_id" : "5c92b80436de59bd9de0639e" }
{ "_id" : "5c92b80936de59bd9de0639f" }
{ "_id" : "5c92b81836de59bd9de063a0" }为了获得原始的ObjectId，请使用$toObjectId运算符-
> db.objectidToStringDemo.aggregate([ { $project: { _id: { $toObjectId: "$_id" } } } ] );以下是输出-
{ "_id" : ObjectId("5c92b80036de59bd9de0639d") }
{ "_id" : ObjectId("5c92b80436de59bd9de0639e") }
{ "_id" : ObjectId("5c92b80936de59bd9de0639f") }
{ "_id" : ObjectId("5c92b81836de59bd9de063a0") }