committee_spec.rb

describe Committee do  
  it { should link_one(:municipality) }  
end

supplemental_matchers.rb

Rspec::Matchers.define :link_one do |expected|
  match do |actual|
    (actual.should include_association(expected)) &&
    (actual.should associate_one(expected))
  end
end

# "lower-level" matchers
Rspec::Matchers.define :associate_one do |expected|
  match do |actual|
    actual.class.associations[expected.to_sym].try(:one?)
  end
  failure_message_for_should do |actual|
    "expected #{ actual.class.associations[expected.to_sym].inspect} to associate one"
  end
  failure_message_for_should_not do |actual|
    "expected #{ actual.class.associations[expected.to_sym].inspect} to not associate one"
  end
end

Rspec::Matchers.define :include_association do |expected|
  match do |actual|
    actual.class.associations.keys.map(&:to_sym).include?(expected.to_sym)
  end
  failure_message_for_should do |actual|
    "expected #{actual.class.associations.keys.map(&:to_sym)} to include #{expected.to_sym}"
  end
  failure_message_for_should_not do |actual|
    "expected #{actual.class.associations.keys} to not include #{expected}"
  end
end