如何确定MongoDB中是否存在字段？
您需要使用$exists运算符来确定MongoDB中是否存在字段。首先让我们创建一个包含文档的集合
> db.determineFieldExistsDemo.insertOne({"ClientName":"John"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c9eb245d628fa4220163b75")
}
> db.determineFieldExistsDemo.insertOne({"ClientName":"Larry","ClientAge":23});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c9eb25cd628fa4220163b76")
}
> db.determineFieldExistsDemo.insertOne({"ClientName":"Mike","ClientCountryName":"UK"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c9eb26fd628fa4220163b77")
}
> db.determineFieldExistsDemo.insertOne({"ClientName":"Sam","ClientAge":24});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c9eb286d628fa4220163b78")
}以下是在find()方法的帮助下显示集合中所有文档的查询
> db.determineFieldExistsDemo.find().pretty();
这将产生以下输出
{ "_id" : ObjectId("5c9eb245d628fa4220163b75"), "ClientName" : "John" }
{
"_id" : ObjectId("5c9eb25cd628fa4220163b76"),
"ClientName" : "Larry",
"ClientAge" : 23
}
{
"_id" : ObjectId("5c9eb26fd628fa4220163b77"),
"ClientName" : "Mike",
"ClientCountryName" : "UK"
}
{
"_id" : ObjectId("5c9eb286d628fa4220163b78"),
"ClientName" : "Sam",
"ClientAge" : 24
}以下是查询以确定是否存在字段
> db.determineFieldExistsDemo.find({ClientCountryName:{$exists:true}}).pretty();这将产生以下输出
{
"_id" : ObjectId("5c9eb26fd628fa4220163b77"),
"ClientName" : "Mike",
"ClientCountryName" : "UK"
}