如何查找文档-MongoDB中是否存在元素？
> db.demo497.insertOne({"details":[{"Name":"Chris"},{"Name":"Bob"}]});{
"acknowledged" : true,
"insertedId" : ObjectId("5e84b3cfb0f3fa88e22790d1")
}
> db.demo497.insertOne({"details":[{"Name":"Carol"}]});{
"acknowledged" : true,
"insertedId" : ObjectId("5e84b3d9b0f3fa88e22790d2")
}
> db.demo497.insertOne({"details":[{}]});{
"acknowledged" : true,
"insertedId" : ObjectId("5e84b3e9b0f3fa88e22790d3")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo497.find();
这将产生以下输出-
{ "_id" : ObjectId("5e84b3cfb0f3fa88e22790d1"), "details" : [ { "Name" : "Chris" }, { "Name" : "Bob" } ] }
{ "_id" : ObjectId("5e84b3d9b0f3fa88e22790d2"), "details" : [ { "Name" : "Carol" } ] }
{ "_id" : ObjectId("5e84b3e9b0f3fa88e22790d3"), "details" : [ { } ] }以下是查找元素是否存在于文档中的查询-
> db.demo497.find({$or:[
... {"details.Name" : {$ne : "Carol"}},
... {details: {$exists: false}}
... ]
... });这将产生以下输出-
{ "_id" : ObjectId("5e84b3cfb0f3fa88e22790d1"), "details" : [ { "Name" : "Chris" }, { "Name" : "Bob" } ] }
{ "_id" : ObjectId("5e84b3e9b0f3fa88e22790d3"), "details" : [ { } ] }