The best replacement for datamapper is sequel.

The best alternatives to datamapper are rated as following:
★★★★★  sequel
★★★★★  activerecord
★★★  activerecord sinatra
★★★  repository pattern
★★  doctrine
  datamapper

Your alternative for datamapper is not listed? Suggest!
Speaking about datamapper, one probably shouldn't forget about sinatra.

Other search terms related to datamapper and their ratings are:
★★★★★ sinatra
★★★★ rails 3
You have in mind a companion for datamapper and it is not yet here? Propose!



Google Trends

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datamapper
sequel
activerecord