update array of objects with an array in a mongodb aggregation - mongodb

Question

I have the following situation in my aggregation: At some point my documents are holding an array of objects and an array of the same length at the root level. An example would be:
{_id:0,
cars:[{name:"vw polo",door:3},{name:"vw golf",door:5}],
possible_colors:[["black","blue"],["white"]]}
What I'm trying now is to update project the possible colors into each object of the cars array. The expected result should look as follows.
{_id:0,
cars:[{name:"vw polo",door:3,possible_colors:["black","blue"]},
{name:"vw golf",door:5,possible_colors:["white"]}],
}
I already tried {$addfields:{cars:{$zip:[$cars,$possible_colors]}}}, but this creates a list of arrays, where each array contains the object and the correct subarray, but i was not able to merge them.

Answer 1

The $zip operator is a good approach however you need additional $map along with $mergeObjects in order to get the desired structure:
db.collection.aggregate([
{
$addFields: {
cars: {
$map: {
input: { $zip: { inputs: [ "$cars", "$possible_colors" ] } },
in: {
$mergeObjects: [
{ $arrayElemAt: [ "$$this", 0 ] },
{ possible_colors: { $arrayElemAt: [ "$$this", 1 ] } }
]
}
}
}
}
}
])
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