仅在缺少MongoDB文档字段时才插入？
首先让我们创建一个包含文档的集合-
>db.missingDocumentDemo.insertOne({"StudentFirstName":"Adam","StudentLastName":"Smith"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd3fb1eedc6604c74817ce6")
}
>db.missingDocumentDemo.insertOne({"StudentFirstName":"Carol","StudentLastName":"Taylor"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd3fb29edc6604c74817ce7")
}
>db.missingDocumentDemo.insertOne({"StudentFirstName":"David","StudentLastName":"Miller","StudentAge":21});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd3fb40edc6604c74817ce8")
}以下是在find()方法的帮助下显示集合中所有文档的查询-
> db.missingDocumentDemo.find().pretty();
这将产生以下输出-
{
"_id" : ObjectId("5cd3fb1eedc6604c74817ce6"),
"StudentFirstName" : "Adam",
"StudentLastName" : "Smith"
}
{
"_id" : ObjectId("5cd3fb29edc6604c74817ce7"),
"StudentFirstName" : "Carol",
"StudentLastName" : "Taylor"
}
{
"_id" : ObjectId("5cd3fb40edc6604c74817ce8"),
"StudentFirstName" : "David",
"StudentLastName" : "Miller",
"StudentAge" : 21
}这是仅在缺少MongoDB文档字段时才插入的查询。我们正在尝试在此处插入StudentAge字段。如果该字段已经存在，则不会插入该字段-
> db.missingDocumentDemo.update(
... { "StudentAge": { "$exists": false } },
... { "$set": { "StudentAge": 23 } },
... { "multi": true }
... );
WriteResult({ "nMatched" : 2, "nUpserted" : 0, "nModified" : 2 })让我们显示上述集合中的所有文档-
> db.missingDocumentDemo.find().pretty();
这将产生以下输出-
{
"_id" : ObjectId("5cd3fb1eedc6604c74817ce6"),
"StudentFirstName" : "Adam",
"StudentLastName" : "Smith",
"StudentAge" : 23
}
{
"_id" : ObjectId("5cd3fb29edc6604c74817ce7"),
"StudentFirstName" : "Carol",
"StudentLastName" : "Taylor",
"StudentAge" : 23
}
{
"_id" : ObjectId("5cd3fb40edc6604c74817ce8"),
"StudentFirstName" : "David",
"StudentLastName" : "Miller",
"StudentAge" : 21
}