MongoDB AND Comparison Fails - mysql

Question

I have a Collection named StudentCollection with two documents given below,
> db.studentCollection.find().pretty()
{
"_id" : ObjectId("52d7c0c744b4dd77efe93df7"),
"regno" : 101,
"name" : "Ajeesh",
"gender" : "Male",
"docs" : [
"voterid",
"passport",
"drivinglic"
]
}
{
"_id" : ObjectId("52d7c6a144b4dd77efe93df8"),
"regno" : 102,
"name" : "Sathish",
"gender" : "Male",
"dob" : ISODate("2013-12-09T21:05:00Z")
}
Why does the below query returns a document when it doesn't fulfil the criteria which I gave in find command. I know it's a bad & stupid query for AND comparison. I tried this with MySQL and it doesn't return anything as expected but why does NOSQL makes problem. I hope it's considering the last field for comparison.
> db.studentCollection.find({regno:101,regno:102}).pretty()
{
"_id" : ObjectId("52d7c6a144b4dd77efe93df8"),
"regno" : 102,
"name" : "Sathish",
"gender" : "Male",
"dob" : ISODate("2013-12-09T21:05:00Z")
}
Can anyone brief why does Mongodb works this way?

Answer 1

MongoDB leverages JSON/BSON and names should be unique (http://www.ietf.org/rfc/rfc4627.txt # 2.2.) Found this in another post How to generate a JSON object dynamically with duplicate keys? . I am guessing the value for 'regno' gets overridden to '102' in your case.
If what you want is an OR query, try the following:
db.studentCollection.find ( { $or : [ { "regno" : "101" }, {"regno":"102"} ] } );
Or even better, use $in:
db.studentCollection.find ( { "regno" : { $in: ["101", "102"] } } );
Hope this helps!
Edit : Typo!

Answer 2

MongoDB converts your query to a Javascript document. Since you have not mentioned anything for $and condition in your document, your query clause is getting overwritten by the last value which is "regno":"102". Hence you get last document as result.
If you want to use an $and, you may use any of the following:
db.studentCollection.find({$and:[{regno:"102"}, {regno:"101"}]});
db.studentCollection.find({regno:{$gte:"101", $lte:"102"}});