Improve failure message when has_many is missing

When test driving, writing an expectation such as

```ruby
it { should have_many(:children).through(:relationships).source(:child) }
```

used to provide the not-so-helpful error message

```
Failure/Error: it { should have_many(:children).through(:relationships).source(:child) }

     NoMethodError:

       undefined method `options' for nil:NilClass
```

The improved error message is

```
Expected Parent to have a has_many association called children (no association called children)
```

NEWS.md

@@ -9,8 +9,13 @@
   another validation on the same attribute, the matcher does not fail with an
   error. ([#592])
 
+* Fix `has_many` used with `through` so that when the association does not
+  exist, and the matcher fails, it does not raise an error when producing the
+  failure message. ([#588])
+
 [#591]: https://github.com/thoughtbot/shoulda-matchers/pull/591
 [#592]: https://github.com/thoughtbot/shoulda-matchers/pull/592
+[#588]: https://github.com/thoughtbot/shoulda-matchers/pull/588
 
 # 2.7.0
 

lib/shoulda/matchers/active_record/association_matcher.rb

@@ -889,7 +889,7 @@ module Shoulda
         end
 
         def missing_options
-          missing_options = [missing, failing_submatchers.map(&:missing_option)]
+          missing_options = [missing, missing_options_for_failing_submatchers]
           missing_options.flatten.compact.join(', ')
         end
 
@@ -899,6 +899,14 @@ module Shoulda
           end
         end
 
+        def missing_options_for_failing_submatchers
+          if defined?(@failing_submatchers)
+            @failing_submatchers.map(&:missing_option)
+          else
+            []
+          end
+        end
+
         def association_exists?
           if reflection.nil?
             @missing = "no association called #{name}"

spec/shoulda/matchers/active_record/association_matcher_spec.rb

@@ -325,6 +325,16 @@ describe Shoulda::Matchers::ActiveRecord::AssociationMatcher do
       expect(matcher.failure_message).to match(/through relationships, but got it through conceptions/)
     end
 
+    it 'produces a failure message without exception when association is missing :through option' do
+      define_model :child
+      define_model :parent
+      matcher = have_many(:children).through(:relationships).source(:child)
+      failure_message = 'Expected Parent to have a has_many association called children (no association called children)'
+
+      matcher.matches?(Parent.new)
+      expect(matcher.failure_message).to eq failure_message
+    end
+
     it 'accepts an association with a valid :dependent option' do
       expect(having_many_children(dependent: :destroy)).
         to have_many(:children).dependent(:destroy)