MongoDB find all documents where an array / list size is greater than N

Review following documents :

	"_id" : 1001,
	"domainName" : "google.com"
	"tag" : [
		"search engine",
		"search",
		"find anything",
		"giant"
},
	"_id" : 1002,
	"domainName" : "yahoo.com"
	"tag" : [
		"search engine",
		"online portal",
},
	"_id" : 1003,
	"domainName" : "mkyong.com"

1. Question

How to find all documents where “tag” size is greater than 3?

#1.1 Try combine $size and $gt like this :

db.domain.find( { tag: {$size: {$gt:3} } } );
null

No error, but it will always return a null, seem MongoDB doesn’t works like this.

#1.2 Try use $where operator.

db.domain.find( {$where:'this.tag.length>3'} )
MongoDB v 2.2.3
uncaught exception: error {
	"$err" : "error on invocation of $where function:\nJS Error: TypeError: this.tag has no properties nofile_a:0",
	"code" : 10071
or 
MongoDB v 2.4.5
JavaScript execution failed: error: {
        "$err" : "JavaScript execution failed: TypeError: Cannot read property 'length' of undefined
 near '' ",
        "code" : 16722
} at src/mongo/shell/query.js:L128
>

Look like the $where operator is not working as well?

2. Solution

Actually, the $where operator is working fine, just not all documents contains the “tag” field, and caused the “no properties” error.

2.1 To fix it, try combine $exists and $where together :

db.domain.find( {tag : {$exists:true}, $where:'this.tag.length>3'} )
	"_id" : 1001,
	"domainName" : "google.com"
	"tag" : [
		"search engine",
		"search",
		"find anything",
		"giant"

References

1. MongoDB $where
2. MongoDB $exists
3. Stackoverflow : how do I find documents where array size is greater than 1
4. MongoDB $where clause to query array length