fixes #9025 - reflect on association for _name(s) methods

[stbenjam]

The current method doesn't work for associations in another module (like in plugins).

In the Salt plugin, some_host.salt_environment_name resolves to SaltEnvironment instead of ForemanSalt::SaltEnvironment. This PR changes how the class is resolved, by instead relying on active record reflection.

[dLobatog]

@stbenjam Thanks, works well (I tried with foreman_salt). Can you write a test with a dummy class demonstrating it?

[stbenjam]

@elobato Added a test... Testing that is actually a bit convoluted, and has_many_common.rb already has some tests that validate it's at least working correctly (for Foreman, anyway).

We need another gem called Temping in order to really test the association bits on a plugin model. I have it working, it's maybe a bit too clever?

[dLobatog]

How about:

  class ::FakePlugin; end 
  class ::FakePlugin::FakeModel 
  end

  test "should return plugin associations" do
    Host::Managed.class_eval do
      belongs_to :fake_model, :class_name => '::FakePlugin::FakeModel'
    end 
    assert_equal FactoryGirl.build(:host).assoc_klass(:fake_model), FakePlugin::FakeModel
  end 

❓

[stbenjam]

@elobato Updated. Thanks, I've used your test and it's 🍏 👍

[dLobatog]

Merged as 1cf860b, thanks @stbenjam !