Database
[{"_id":{"$oid":"62df5769b6f6345d3e91130d"},person_id:ObjectId("62df5722d361708c1f0a7e9f"),"user_groups":[{"id":{"$oid":"62df5769d361708c1f0a7ea1"}}]},{"_id":{"$oid":"62df584eb6f6345d3e91138a"},person_id:ObjectId("62df5722d361708c1f0a7e9f"),"user_groups":[{"$oid":"62df5769d361708c1f0a7ea1"}]}]
Query
db.collection.aggregate([{$match:{person_id:ObjectId("62df5722d361708c1f0a7e9f")}},{$project:{user_groups:{$map:{input:"$user_groups",in:{$ifNull:["$$this.id","$$this"]}}}}},])