Upsert hash in mongodb. Remove unused keys - ruby

Question

From the Mongo documentation:
If you specify multiple field-value pairs, $set will update or create
each field.
I have a mongoid document like this:
class MyCounter
include Mongoid::Document
field :date, type: Date
field :properties, type: Hash
end
and when I try to change the properties like this:
hash = {properties.0 => 50, properties.1 => 100 }
MyCounter.where(something).find_and_modify(
{ '$set' => hash, {'upsert' => 'true', new: true}
)
it keeps the old keys on the property hash.
What's the correct way to completely replace (and create a new document if it doesn't exist) an hash in a document?
EDIT
I'm currently doing this which is dumb:
MyCounter.where(
date: date
).find_and_modify(
{ '$unset' => { properties: nil} }, {'upsert' => 'true', new: true}
)
MyCounter.where(
date: date
).find_and_modify(
{ '$set' => hash }, {'upsert' => 'true', new: true}
)

Answer 1

Just don't use $set. By only passing field: value pairs all the fields are replaced (except the _id field)
This should work fine.
MyCounter.where(
date: date
).find_and_modify(
hash, {'upsert' => 'true', new: true}
)
http://docs.mongodb.org/manual/reference/method/db.collection.update/#example-update-replace-fields