MongoDB: how to combine two different data structure to one - mongodb

Question

Here is my collection:
db.sites_people.find({person_id: ObjectId('62df5722d361708c1f0a7e9f')}, {user_groups: 1}):
[
{
'_id': { '$oid': '62df5769b6f6345d3e91130d' },
'user_groups': [
{
'id': { '$oid': '62df5769d361708c1f0a7ea1' },
},
],
},
{
'_id': { '$oid': '62df584eb6f6345d3e91138a' },
'user_groups': [
{ '$oid': '62df5769d361708c1f0a7ea1' },
],
},
];
As you can see, there are two different-structure under user_groups .
I try to use $ifNull to combine the user_groups, but it's not work.
db.sites_people.aggregate([
{ $match: { person_id: ObjectId('62df5722d361708c1f0a7e9f') } },
{ $project: { user_groups: { $ifNull: ['$user_groups.id', '$user_groups'] } } },
]);
[
{
'_id': { '$oid': '62df5769b6f6345d3e91130d' },
'user_groups': [
{ '$oid': '62df5769d361708c1f0a7ea1' },
],
},
{
'_id': { '$oid': '62df584eb6f6345d3e91138a' },
'user_groups': [],
},
];
How can I get the result like this:
[
{
'_id': { '$oid': '62df5769b6f6345d3e91130d' },
'user_groups': [
{ '$oid': '62df5769d361708c1f0a7ea1' },
],
},
{
'_id': { '$oid': '62df584eb6f6345d3e91138a' },
'user_groups': [
{ '$oid': '62df5769d361708c1f0a7ea1' },
],
},
];

Answer 1

Best way is to just use $map to convert the arrays, this will also allow you to have unified structures for mixed type arrays ( one element with id and one without ), like so:
db.sites_people.aggregate([
{
$match: {
person_id: ObjectId("62df5722d361708c1f0a7e9f")
}
},
{
$project: {
user_groups: {
$map: {
input: "$user_groups",
in: {
$ifNull: [
"$$this.id",
"$$this"
]
}
}
}
}
}
])
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