MongoDB：对过滤字段使用值1

Question

I am working on a existing code in which they are using Mongo DB with Java EE.

Under which in some case for filteration purposes , they are using the hard coded value as 1 .

BasicDBObject query = new BasicDBObject(SEC_TYPE, "A");

BasicDBObject Fields = new BasicDBObject("date", 1);
returnFields.put("unisymbol", 1);
returnFields.put("symbol", 1);


DBCursor cursor = coll.find(query, Fields);

For example if you see the above query the Fields such as date , unisymbol and symbol are hardcoded to 1 .

Please let me know what does , putting the value "1" aganist the filed mean exactly ??

Answer 1

It means, "retrieve value only for this field". So, this query:

db.users.find({}, {firstname: 1, lastname: 1})

will get only two fields, first name and last name. It won't get email, address and other fields.

Answer 2

It is used for projection: http://docs.mongodb.org/manual/core/read-operations/#result-projections whereby the fields of the document are being filtered out. In this case 1 against that field means to include it and 0 mens to omit it.

So the code above includes date , unisymbol , symbol and _id . It includes the _id since that has to be explicitly removed from the result via:

returnFields.put("_id", 0);