module ActiveSupport::Testing::Assertions

assert_not foo, 'foo should be false'

An error message can be specified.

assert_not 'foo' # => Expected "foo" to be nil or false
assert_not false # => true
assert_not nil # => true

like if foo.
+nil+ or +false+. "Truthy" means "considered true in a conditional"
Asserts that an expression is not truthy. Passes if object is

def assert_not(object, message = nil)
  message ||= "Expected #{mu_pp(object)} to be nil or false"
  assert !object, message
end