This code snippet is designed to maintain the order of an array (var list) when utilizing mongoDb's $in clause effectively.
Although, I must admit that the logic behind it is not entirely clear to me.
I can see that it's iterating in reverse to preserve the order, but why is ref being assigned to list at i - 1, rather than i?
It's a bit challenging for someone like me with basic javascript knowledge, but it's a good exercise to test my current skill level.
If someone could kindly walk me through the process step-by-step, I would greatly appreciate it.
var list = [ 4, 2, 8 ];
var stack = [];
for (var i = list.length - 1; i > 0; i--) {
var rec = {
"$cond": [
{ "$eq": [ "$_id", list[i-1] ] },
i
]
};
if ( stack.length == 0 ) {
rec["$cond"].push( i+1 );
} else {
var lval = stack.pop();
rec["$cond"].push( lval );
}
stack.push( rec );
}
var pipeline = [
{ "$match": { "_id": { "$in": list } }},
{ "$project": { "weight": stack[0] }},
{ "$sort": { "weight": 1 } }
];
db.collection.aggregate( pipeline );