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 the results.
• and if such field doesn't exist, it will also add document to the results?

In the end we will receive all documents where certain field have matched condition AND all document where such field doesn't exist at all.

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.