Add failing test for double exposures #56

[Morred]

#56

[marshall-lee]

Hello, @Morred!

I closed & reopened your PR just to initiate a re-run of Travis CI checks. Build still fails but this time it's just due to RuboCop offenses. RSpec tests pass fine as you can see here: https://travis-ci.org/intridea/grape-entity/jobs/73909288 so issue #56 can be considered as solved.

I will not merge this PR because new specs from #151 for this issue are sufficient. If you don't think so and want this to be merged please somehow fix RuboCop offenses.

spec/grape_entity/entity_spec.rb:338:52: C: Use the new lambda literal syntax ->(params) {...}.

            expose :media, using: SoundEntity, if: lambda { |object, _| object.sound? }

                                                   ^^^^^^

spec/grape_entity/entity_spec.rb:339:52: C: Use the new lambda literal syntax ->(params) {...}.

            expose :media, using: PhotoEntity, if: lambda { |object, _| object.photo? }

                                                   ^^^^^^

I personally don't like this type of RuboCop offense about lambda syntax and if you think the same feel free to open a separate PR with patch for .rubocop.yml.

I also don't like defining constants like PhotoEntity in specs because these things are global and survive among rspec examples. I know there are some specs in entity_spec.rb that already define constants but it's something that I want to get rid of. I already had issues with constants while working on this project. @dblock, what do you think? Also I need to say that this problem is gracefully solved in https://github.com/Casecommons/with_model gem but it's only for ActiveRecord models. I also seen a good workaround for this in Her gem, please look at https://github.com/remiprev/her/blob/49cd15ffbad2aec104810753a90d9a7b6977fd31/spec/support/macros/model_macros.rb#L6 and https://github.com/remiprev/her/blob/c39fbb6d211109da5f2ff8c35fff4b5a85d8d5c9/spec/spec_helper.rb#L21