How do I write the following Mongo Aggregation Query using mgo with Golang

I have the following query which I have tested and works, but the mgo

var userId = "57a944390b1acf0d069388c1";
db.users.aggregate([
        { "$match": { "_id": userID } },
        { "$unwind": "$groups" },
        {
            "$lookup": {
                "from": "groups",
                "localField": "groups.id",
                "foreignField": "_id",
                "as": "group"
            }
        },
        { "$unwind": "$group" },
        {
            "$project": {
                "group.requests": {
                	"$filter": {
                		"input" : "$group.requests",
                		"as" : "item",
                		"cond": { "$and" : [
                				{ "$ne" : ["$$item.user_id", userID] },
                				{ "$not" : {
                						"$setIsSubset" : [
                							[userID], "$$item.denied_users"
                						]
                					}
                				}
                			]     			
                		}
                	}
                }
            }
        },
        { "$unwind" : "$group.requests" }
      ])

The mgo query looks like the following:

c := session.DB(info.Db()).C("users")

	o1 := bson.M{"$match": bson.M{"_id": userID}}
	o2 := bson.M{"$unwind": "$groups"}
	o3 := bson.M{"$lookup": bson.M{
		"from":         "groups",
		"localField":   "groups.id",
		"foreignField": "_id",
		"as":           "group",
	}}
	o4 := bson.M{"$unwind": "$group"}
	o5 := bson.M{
		"$project": bson.M{
			"group.requests": bson.M{
				"$filter": bson.M{
					"input": "$group.requests",
					"as":    "item",
					"cond": bson.M{
						"$and": []bson.M{
							bson.M{"$ne": []string{"$$item.user_id", userID}},
							bson.M{"$not": bson.M{
								"$setIsSubset": []interface{}{
									[]string{userID},
									[]string{"$$item.denied_users"},
								},
							}},
						},
					},
				},
			},
		},
	}
	o6 := bson.M{"$unwind": "$group.requests"}
	pipeline := []bson.M{o1, o2, o3, o4, o5, o6}

My guess is that the $filter is not working. Specifically, the way I am specifying the $setIsSubset and arguments to that are not matching the actual mongo query above it. If I remove that section (everything within the $not, then it works (aside from not being the correct filter).

I basically need the top query translated into mgo.

"$setIsSubset": []interface{}{
    []string{userID},
    []string{"$$item.denied_users"},
},

is equal to

"$setIsSubset" : [
    [userID], ["$$item.denied_users"]
]

so I think you need to define it like

"$setIsSubset": []interface{}{
    []string{userID},
    "$$item.denied_users",
},