检查MongoDB是否存在该字段？
您可以为此使用$exists和$ne运算符。为了进一步理解该概念，让我们创建一个带有文档的集合。创建带有文档的集合的查询如下-
> db.checkFieldExistDemo.insertOne({"EmployeeId":1,"EmployeeName":"John","isMarried":true,"EmployeeSalary":4648585});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c76f7b31e9c5dd6f1f78281")
}
> db.checkFieldExistDemo.insertOne({"StudentId":2,"StudentName":"John","isMarried":false," StudentAge":19});
{
"acknowledged" : true,0
"insertedId" : ObjectId("5c76f7e11e9c5dd6f1f78282")
}在find()method的帮助下显示集合中的所有文档。查询如下-
> db.checkFieldExistDemo.find().pretty();
输出结果
{
"_id" : ObjectId("5c76f7b31e9c5dd6f1f78281"),
"EmployeeId" : 1,
"EmployeeName" : "John",
"isMarried" : true,
"EmployeeSalary" : 4648585
}
{
"_id" : ObjectId("5c76f7e11e9c5dd6f1f78282"),
"StudentId" : 2,
"StudentName" : "John",
"isMarried" : false,
"StudentAge" : 19
}这是检查该字段在MongoDB中是否存在的查询。
情况1-当一个字段存在于多个文档中时。查询如下-
> db.checkFieldExistDemo.find({"isMarried":{$exists:true,$ne:null}}).pretty();输出结果
{
"_id" : ObjectId("5c76f7b31e9c5dd6f1f78281"),
"EmployeeId" : 1,
"EmployeeName" : "John",
"isMarried" : true,
"EmployeeSalary" : 4648585
}
{
"_id" : ObjectId("5c76f7e11e9c5dd6f1f78282"),
"StudentId" : 2,
"StudentName" : "John",
"isMarried" : false,
"StudentAge" : 19
}情况2-如果一个文档中只有一个字段。查询如下-
> db.checkFieldExistDemo.find({"StudentName":{$exists:true,$ne:null}}).pretty();输出结果
{
"_id" : ObjectId("5c76f7e11e9c5dd6f1f78282"),
"StudentId" : 2,
"StudentName" : "John",
"isMarried" : false,
"StudentAge" : 19
}