input.json

{"Type":"post","ID":1,"Thread":1,"Modified":{"Count":1,"Last":"2013-05-26T02:35:22.422030759Z"},"Created":"2013-05-26T02:35:22.422030759Z","Addr":"[::1]:44608","Title":"Test","Content":"\u003cem\u003etest\u003c/em\u003e"}

map.js

function( doc, meta ) {
	if ( doc.Type == "post" ) {
		emit( doc.Thread, null );
	}
}

output.json

{"total_rows":0,"rows":[]}