我想在已经创建的文档中创建一个新字段。如何使用MongoDB查询做到这一点？
使用$addToSet在MongoDB中创建一个新字段。首先让我们创建一个包含文档的集合-
> db.createFieldDemo.insertOne({"StudentFirstName":"John","StudentAge":21});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd99e28b50a6c6dd317ad95")
}
> db.createFieldDemo.insertOne({"StudentFirstName":"Larry","StudentAge":23});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd99e2fb50a6c6dd317ad96")
}
> db.createFieldDemo.insertOne({"StudentFirstName":"Chris","StudentAge":22});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd99e38b50a6c6dd317ad97")
}
> db.createFieldDemo.insertOne({"StudentFirstName":"David","StudentAge":25});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd99e43b50a6c6dd317ad98")
}以下是在find()方法的帮助下显示集合中所有文档的查询-
> db.createFieldDemo.find().pretty();
这将产生以下输出-
{
"_id" : ObjectId("5cd99e28b50a6c6dd317ad95"),
"StudentFirstName" : "John",
"StudentAge" : 21
}
{
"_id" : ObjectId("5cd99e2fb50a6c6dd317ad96"),
"StudentFirstName" : "Larry",
"StudentAge" : 23
}
{
"_id" : ObjectId("5cd99e38b50a6c6dd317ad97"),
"StudentFirstName" : "Chris",
"StudentAge" : 22
}
{
"_id" : ObjectId("5cd99e43b50a6c6dd317ad98"),
"StudentFirstName" : "David",
"StudentAge" : 25
}以下是创建新字段的查询。在这里，我们创建一个字段“StudentLastName”-
> db.createFieldDemo.update({_id: ObjectId("5cd99e43b50a6c6dd317ad98")}, {$addToSet: {"StudentLastName": "Miller"}});
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })让我们检查上述集合中的所有文档-
> db.createFieldDemo.find().pretty();
这将产生以下输出-
{
"_id" : ObjectId("5cd99e28b50a6c6dd317ad95"),
"StudentFirstName" : "John",
"StudentAge" : 21
}
{
"_id" : ObjectId("5cd99e2fb50a6c6dd317ad96"),
"StudentFirstName" : "Larry",
"StudentAge" : 23
}
{
"_id" : ObjectId("5cd99e38b50a6c6dd317ad97"),
"StudentFirstName" : "Chris",
"StudentAge" : 22
}
{
"_id" : ObjectId("5cd99e43b50a6c6dd317ad98"),
"StudentFirstName" : "David",
"StudentAge" : 25,
"StudentLastName" : [
"Miller"
]
}