Rails ActiveSupport: How to assert that an error is raised?

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So unit testing isn’t really in activesupport. Ruby comes with a typical xunit framework in the standard libs (Test::Unit in ruby 1.8.x, MiniTest in ruby 1.9), and the stuff in activesupport just adds some stuff to it.

If you are using Test::Unit/MiniTest 

assert_raise(Exception) { whatever.merge }

if you are using rspec (unfortunately poorly documented, but way more popular)

lambda { whatever.merge }.should raise_error

If you want to check the raised Exception:

exception = assert_raises(Exception) { whatever.merge }
assert_equal( "message", exception.message )

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