问题
Is it possible to get the number of fields in document using a Query From MongoDB java Driver
Example:
Document1 :{_id:1,"type1": 10 "type2":30,"ABC":123,"DEF":345}
Document2 :{_id:2,"type2":30,"ABC":123,"DEF":345}
Note: In second document "type1" key doesnt exist .
When i project
Is it possible to project only "type1" and "type2" and get number of fields existing in that document.
With current code i am getting all the documents and individually searching if there is the key i am looking is present in the whole cursor: The code snipped is as follows:
MongoClient mcl=new MongoClient();
MongoDatabase mdb=mcl.getDatabase("test");
MongoCollection mcol=mdb.getCollection("testcol");
FindIterable<Document> findIterable = mcol.find();
MongoCursor<Document> cursor = findIterable.iterator();
//Here am having 120 types checking if each type is present..
while(cursor.hasNext())
{
Document doc=cursor.next();
int numberOfTypes=0;
for(int i=1;i<=120;i++)
{
if(doc.containsKey("type"+i))
{
numberOfTypes++;
}
}
System.out.println("*********************************************");
System.out.println("_id"+doc.get("_id"));
System.out.println("Number of Types in this document are "+numberOfTypes);
System.out.println("*********************************************");
}
}
This code is working if the records are less it wont be over load to the application ..Suppose there are 5000000 with each Document containing 120 types , the application is crashing as there would be more garbage collection involved as for every iteration we are creating a document.Is there any other approach through which we can achieve the above stated functionality.
回答1:
From your java code I read
project only "type1" and "type2" and get number of fields existing in that document.
as
project only type[1..120] fields and number of such fields in the document
With this assumption, you can map-reduce it as following:
db.testcol.mapReduce(
function(){
value = {count:0};
for (i = 1; i <= 120; i++) {
key = "type" + i
if (this.hasOwnProperty(key)) {
value[key] = this[key];
value.count++
}
}
if (value.count > 0) {
emit(this._id, value);
}
},
function(){
//nothing to reduce
},
{
out:{inline:true}
});
out:{inline:true} works for small datasets, when result fits into 16Mb limit. For larger responses you need to output to a collection, which you can query and iterate as usual.
来源：https://stackoverflow.com/questions/35241567/get-the-count-of-fields-in-each-document-through-query-using-mongodb-java-driver