db_populate/lib/create_or_update.rb

class ActiveRecord::Base
  # given a hash of attributes including the ID, look up the record by ID.
  # uses whatever the PK of the model is to do the lookup 
  # If it does not exist, it is created with the rest of the options. 
  # If it exists, it is updated with the given options. 
  #
  # Raises an exception if the record is invalid to ensure seed data is loaded correctly.
  # Pass :perform_validations => false to skip validations in the model.
  #
  # Returns the record.
  def self.create_or_update(options = {})
    id = options.delete(primary_key.to_sym)
    validate = options.delete(:perform_validations){|k| true}
    record = send("find_by_#{primary_key}", id) || new
    record.id = id
    if validate
      record.attributes = options
      record.save!
    else
      options.each do |key, value|
        record.send("#{key}=", value)
      end
      record.save(false)
    end
    record
  end

  # for PostgreSQL (and others?) we must reset the id sequential to the next
  # available highest number. This happens because the create_or_update
  # creates records with an id already set, bypassing PostgreSQL's automatic
  # incrementing sequential id counter
  # Otherwise, we get the dreaded "duplicate key value violates unique constraint" error
  def self.postgresql_reset_id_seq
    connection.execute("SELECT setval('#{self.table_name}_id_seq'::regclass, MAX(id)) FROM #{self.table_name}")
  end
end