I'm looking forward to a solution very similar to the one proposed in this question: Select records matching concat value of two fields in mongodb
I have two fields: firstName and lastName, and I want to search a string that can be contained in this two fields.
In that question I linked before I found this:
db.coll.find({$expr:{$eq:["MY QUERY STRING", {$concat:["$firstName", "$lastName"]}]}})
But this expression check for the exact match. I would like something like Sql: %like% instead of $eq`
So if I have a record like:
{
    "firstName": "Mario",
    "lastName": "Roberto Rossi"
}
I want to find that with a query string: Mario Roberto.
How can I change my query to solve this problem?