Question

Given guid and value. If a record exists with that guid, update its value. Otherwise, create a new record with that guid and value. Either case, return the record so that I can get its primary key id.

How to do this with:

• as less database queries as possible
• as less code as possible

Answer 1

The simplest thing is probably

object = Model.find_or_initialize_by_guid(guid)
object.update_attributes :value => value

Which always does 2 queries (a find and an insert/update)

If you know ahead of time that collisions are unlikely you could do

begin
  Model.create(:guid => guid, :value => value)
rescue ActiveRecord::RecordNotUnique
  # find and update existing record
end

This requires a unique index on the guid column. It requires 3 queries in the update case but only 1 query in the create case.

This all assumes we are talking SQL – other datastores, such as mongodb, have the concept of an upsert (update or insert) operation