An "object mother" is a test fixture replacement.
There are many, many fixture replacements available for Ruby. So many, in fact, that it looks like some sort of obligatory induction ritual that every Ruby programmer must undergo in order to become a fully-fledged developer.
- Factory Girl: http://github.com/thoughtbot/factory_girl
- FixtureReplacement: http://github.com/smtlaissezfaire/fixturereplacement
- Fixjour: http://github.com/nakajima/fixjour
- Machinist: http://github.com/notahat/machinist
- Cranky: http://github.com/ginty/Cranky
- Miniskit: http://gist.github.com/273579
- FixtureScenarios: http://github.com/mojombo/fixture-scenarios
- Object Daddy: http://github.com/flogic/object_daddy
- Fabrication: http://github.com/paulelliott/fabrication
FixtureReplacement was one of the first and I used it since very soon after it came out.
In mid-2010 I started looking into Factory Girl as an alternative, as it seemed to be one of the most complete solutions, with a lot of support of tricky edge cases. Soon after I switched over to it definitively.
Machinist looks to have a nicer-looking, more elegant DSL than Factory Girl.