Find all docs where field doesn't exists, plus if field exists apply condition

Question

Is there a way to make a query that:

• if certain field exists, it will apply condition on the field and if it pass, it will add return document and add it to th

Answer 1

How about something like this:

db.stackoverflow.find({
  $or: [
    { howmuch: { $exists:false } },
    { howmuch:5 }
  ]})

In the stackoverflow collection, this will find all documents that do not have the howmuch field plus all documents that do have howmuch set to 5.