thoughtbot/shoulda-matchers — pull request #475 by mcmire (2014-04-12 20:51:13+00:00)
Title: Fix `belong_to` used with polymorphic association

This fixes #473.

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For a polymorphic belongs_to association, the association doesn't
actually point to a class, it points to a concept, so we can't check
that the model it points to exists.