A useful feature for me would be being able to offset the starting value of serial properties.

Use Case:
In my current project we use subsequent ids to track one of our datamapper classes however we need to start these at a specific value for backwards compatibility (so that these ids do not overlap ids previously assigned).

Example:
- Invoice numbers. These almost never start at zero (who wants to look like they've never had a client before!?). - Product numbers - Order numbers - etc.

Current Workaround:

Please note this workaround has serious limitations: it is by no means efficient since it requires a MAX sql call before creating each instance and is does not absolutely ensure values in the database are sequential (a programmer could easily create a instance with a specific id). However it does demonstrate the level of cludge necessary at the moment:

class myModel

include DataMapper::Resource

@@id_starting_value = 1000
@@new_id = lambda do |r, p|
    max_id = Order.max(:id).to_i
    puts max_id
    puts max_id > @@id_starting_value
    puts   (max_id>@@id_starting_value? (max_id+1) : @@id_starting_value)
    return (max_id>@@id_starting_value? (max_id+1) : @@id_starting_value)
end

property :id, Integer, :key => true, :required => true, :default => @@new_id

end

Proposed API Change:

To accomplish the same functionality I propose the following optional attribute be added to Serial Properties:

property :id, Serial, :start_at => 1000

The start_at attribute could also make one call to MAX during the first instance creation and increment that value for the duration of the run.