How to sort fields after applying $setUnion mongoDB?

Question

JavaScript

There is a collection:

{
    "_id" : ObjectId("5338ec2a5b5b71242a1c911c"),
    "people" : [ 
        {
            "name" : "Vasya"
        }, 
        {
            "age" : "30"
        }, 
        {
            "weight" : "80"
        }
    ],
    "animals" : [ 
        {
            "dog" : "Sharick"
        }, 
        {
            "cat" : "Barsik"
        }, 
        {
            "bird" : "parrot"
        }
    ]
},
{
    "_id" : ObjectId("5338ec7f5b5b71242a1c911d"),
    "people" : [ 
        {
            "name" : "Max"
        }, 
        {
            "age" : "32"
        }, 
        {
            "weight" : "78"
        }
    ],
    "animals" : [ 
        {
            "dog" : "Borbos"
        }, 
        {
            "cat" : "Murka"
        }, 
        {
            "bird" : "Eagle"
        }
    ]
}

Query:
db.tmp.aggregate({$project:{"union":{$setUnion:["$people","$animals"]}}})
and result:

{
    "result" : [ 
        {
            "_id" : ObjectId("5338ec2a5b5b71242a1c911c"),
            "union" : [ 
                {
                    "bird" : "parrot"
                }, 
                {
                    "name" : "Vasya"
                }, 
                {
                    "age" : "30"
                }, 
                {
                    "weight" : "80"
                }, 
                {
                    "dog" : "Sharick"
                }, 
                {
                    "cat" : "Barsik"
                }
            ]
        }, 
        {
            "_id" : ObjectId("5338ec7f5b5b71242a1c911d"),
            "union" : [ 
                {
                    "name" : "Max"
                }, 
                {
                    "age" : "32"
                }, 
                {
                    "weight" : "78"
                }, 
                {
                    "bird" : "Eagle"
                }, 
                {
                    "dog" : "Borbos"
                }, 
                {
                    "cat" : "Murka"
                }
            ]
        }
    ],
    "ok" : 1
}

How can I make sure that the fields of each entry in the result array are displayed in the same order, and not out of order?