Slava191

var a = 12;
db.talks.aggregate([
	{$project:{"_id":1,"from":1, "to":1, "msg":1, "ft":["$from", "$to"]}},
	{$match:{"ft":a}},
	{$unwind: "$ft"},	
	{$match:{"ft":{$ne:a}}},
	{$sort:{"_id":1}},
	{$group: {"_id":"$ft", "orig_id":{$last:"$_id"},"from":{$last:"$from"},
		      "to":{$last:"$to"},"msg":{$last:"$msg"}}},
	{$sort:{"_id":1}},
	{$project:{"_id":"$orig_id","from":1, "to":1, "msg":1}}
]);