如何使用MongoDB在数组字段中投影特定元素？
要在数组字段中使用特定于项目的元素，请使用$project。让我们创建一个包含文档的集合-
>db.demo355.insertOne({"id":101,"details":[{"Name":"Chris",isMarried:1},{"Name":"David",isMarried:0},{"Name":"Mike",isMarried:1}]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e568928f8647eb59e5620c5")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo355.find().pretty();
这将产生以下输出-
{
"_id" : ObjectId("5e568928f8647eb59e5620c5"),
"id" : 101,
"details" : [
{
"Name" : "Chris",
"isMarried" : 1
},
{
"Name" : "David",
"isMarried" : 0
},
{
"Name" : "Mike",
"isMarried" : 1
}
]
}以下是对数组字段中特定于项目的元素的查询-
> db.demo355.aggregate([
... {
... $project: {
... details: {
... $filter: {
... input: "$details",
... as: "out",
... cond: { $eq:["$$out.isMarried",1] }
... }
... }
... }
... }
... ])这将产生以下输出-
{
"_id" : ObjectId("5e568928f8647eb59e5620c5"), "details" : [
{ "Name" : "Chris", "isMarried" : 1 },
{ "Name" : "Mike", "isMarried" : 1 }
]
}