MongoDB查询以使用mapReduce()函数显示替代文档并发出偶数字段值
让我们创建一个包含文档的集合-
> db.demo636.insert({id:1});
WriteResult({ "nInserted" : 1 })
> db.demo636.insert({id:2});
WriteResult({ "nInserted" : 1 })
> db.demo636.insert({id:3});
WriteResult({ "nInserted" : 1 })
> db.demo636.insert({id:4});
WriteResult({ "nInserted" : 1 })
> db.demo636.insert({id:5});
WriteResult({ "nInserted" : 1 })
> db.demo636.insert({id:6});
WriteResult({ "nInserted" : 1 })在find()方法的帮助下显示集合中的所有文档-
> db.demo636.find();
这将产生以下输出-
{ "_id" : ObjectId("5e9c127b6c954c74be91e6d2"), "id" : 1 }
{ "_id" : ObjectId("5e9c127e6c954c74be91e6d3"), "id" : 2 }
{ "_id" : ObjectId("5e9c127f6c954c74be91e6d4"), "id" : 3 }
{ "_id" : ObjectId("5e9c12816c954c74be91e6d5"), "id" : 4 }
{ "_id" : ObjectId("5e9c12836c954c74be91e6d6"), "id" : 5 }
{ "_id" : ObjectId("5e9c12896c954c74be91e6d7"), "id" : 6 }以下是实现mapReduce()和仅显示偶数值的查询-
> db.demo636.mapReduce(
... function () {
... oddCounter++;
... var id= this._id;
... delete this._id;
... if ( oddCounter % d != 0 )
... emit(id, this );
... },
... function() {},
... {
... "scope": { "oddCounter": 0, "d": 2 },
... "out": { "inline": 1 }
... }
... )这将产生以下输出-
{
"results" : [
{
"_id" : ObjectId("5e9c127b6c954c74be91e6d2"),
"value" : {
"id" : 1
}
},
{
"_id" : ObjectId("5e9c127f6c954c74be91e6d4"),
"value" : {
"id" : 3
}
},
{
"_id" : ObjectId("5e9c12836c954c74be91e6d6"),
"value" : {
"id" : 5
}
}
],
"timeMillis" : 29,
"counts" : {
"input" : 6,
"emit" : 3,
"reduce" : 0,
"output" : 3
},
"ok" : 1
}