Example of using an anonymous class to build an RSpec custom Matcher

gistfile1.rb

def be_guest_member_of(expected)
  Class.new do
    def initialize(expected)
      @expected = expected
    end

    def matches?(target)
      @target = target
      @target.memberships.where(role: "guest").map(&:network).include? @expected
    end

    def failure_message_for_should
      "expected #{@target.inspect} to be a guest member of #{@expected.inspect}"
    end

    def failure_message_for_should_not
      "expected #{@target.inspect} to not be a guest member of #{@expected.inspect}"
    end
  end.new(expected)
end

# usage example
user1.should be_guest_member_of network2
user1.should_not be_guest_member_of network1

RSpec::Matchers also has a .define that will take care of the config stuff for you. Throw it in spec/support like so:

RSpec::Matchers.define :be_guest_member_of do |expected|
  match do |target|
    target.memberships.where(role: "guest").map(&:network).include? expected
  end

  failure_message_for_should do |target|
    "expected #{target.inspect} to be a guest member of #{expected.inspect}"
  end

  failure_message_for_should_not do |target|
    "expected #{target.inspect} to not be a guest member of #{expected.inspect}"
  end
end

I would argue that this is the cleanest way to go, but make no claims regarding performance.

I do think in this case that reaching through so many associations like that is an indicator that the design should change to improve the tests though.