Fail have_many :through when inverse is invalid

When used with a `:through` association where a corresponding
`belongs_to` association has not been set up on the inverse model, the
`have_many` matcher raises this error:

    Failures:

      1) User should have many projects through clients
         Failure/Error: it {  should have_many(:projects).through :clients }
         NoMethodError:
           undefined method `class_name' for nil:NilClass
         # /Users/<USER>/.rvm/gems/ruby-2.2.0/gems/activerecord-4.2.0/lib/active_record/reflection.rb:871:in `derive_class_name'
         # /Users/<USER>/.rvm/gems/ruby-2.2.0/gems/activerecord-4.2.0/lib/active_record/reflection.rb:147:in `class_name'
         # /Users/<USER>/.rvm/gems/ruby-2.2.0/gems/shoulda-matchers-2.8.0/lib/shoulda/matchers/active_record/association_matcher.rb:1067:in `rescue in class_exists?'
         # /Users/<USER>/.rvm/gems/ruby-2.2.0/gems/shoulda-matchers-2.8.0/lib/shoulda/matchers/active_record/association_matcher.rb:1064:in `class_exists?'
         # /Users/<USER>/.rvm/gems/ruby-2.2.0/gems/shoulda-matchers-2.8.0/lib/shoulda/matchers/active_record/association_matcher.rb:928:in `matches?'
         # ./spec/models/user_spec.rb:5:in `block (2 levels) in <top (required)>'

Fortunately, ActiveRecord has a `check_validity!` method we can call on
an association. For `:through` associations, this will run through a
litany of checks, one of which is to check for the inverse association,
which we want in this case. We rescue the error that is produced and
include this in the failure message.

Co-authored-by: Elliot Winkler <elliot.winkler@gmail.com>

lib/shoulda/matchers/active_record/association_matcher.rb

@@ -1144,6 +1144,7 @@ def matches?(subject)
           @subject = subject
           association_exists? &&
             macro_correct? &&
+            validate_inverse_of_through_association &&
             (polymorphic? || class_exists?) &&
             foreign_key_exists? &&
             primary_key_exists? &&
@@ -1198,7 +1199,14 @@ def macro_description
         end
 
         def expectation
-          "#{model_class.name} to have a #{macro} association called #{name}"
+          expectation =
+            "#{model_class.name} to have a #{macro} association called #{name}"
+
+          if through?
+            expectation << " through #{reflector.has_and_belongs_to_many_name}"
+          end
+
+          expectation
         end
 
         def missing_options
@@ -1241,6 +1249,14 @@ def macro_correct?
           end
         end
 
+        def validate_inverse_of_through_association
+          reflector.validate_inverse_of_through_association!
+          true
+        rescue ::ActiveRecord::ActiveRecordError => error
+          @missing = error.message
+          false
+        end
+
         def macro_supports_primary_key?
           macro == :belongs_to ||
             ([:has_many, :has_one].include?(macro) && !through?)

lib/shoulda/matchers/active_record/association_matchers/model_reflection.rb

@@ -65,16 +65,22 @@ def association_foreign_key
             end
           end
 
+          def validate_inverse_of_through_association!
+            if through?
+              reflection.check_validity!
+            end
+          end
+
+          def has_and_belongs_to_many_name
+            reflection.options[:through]
+          end
+
           protected
 
           attr_reader :reflection, :subject
 
           private
 
-          def has_and_belongs_to_many_name
-            reflection.options[:through]
-          end
-
           def has_and_belongs_to_many_name_table_name
             if has_and_belongs_to_many_reflection
               has_and_belongs_to_many_reflection.table_name

lib/shoulda/matchers/active_record/association_matchers/model_reflector.rb

@@ -4,9 +4,23 @@ module ActiveRecord
       module AssociationMatchers
         # @private
         class ModelReflector
-          delegate :associated_class, :through?, :join_table_name,
-            :association_relation, :polymorphic?, :foreign_key,
-            :association_foreign_key, to: :reflection
+          delegate(
+            :associated_class,
+            :association_foreign_key,
+            :association_relation,
+            :foreign_key,
+            :has_and_belongs_to_many_name,
+            :join_table_name,
+            :polymorphic?,
+            :validate_inverse_of_through_association!,
+            to: :reflection
+          )
+
+          delegate(
+            :through?,
+            to: :reflection,
+            allow_nil: true
+          )
 
           def initialize(subject, name)
             @subject = subject

spec/unit/shoulda/matchers/active_record/association_matcher_spec.rb

@@ -847,6 +847,15 @@ def belonging_to_non_existent_class(model_name, assoc_name, options = {})
       expect(having_many_children).to have_many(:children)
     end
 
+    it 'does not reject a non-:through association where there is no belongs_to in the inverse model' do
+      define_model :Child, parent_id: :integer
+      parent_class = define_model :Parent do
+        has_many :children
+      end
+
+      expect { have_many(:children) }.to match_against(parent_class.new)
+    end
+
     it 'accepts a valid association with a :through option' do
       define_model :child
       define_model :conception, child_id: :integer,
@@ -860,6 +869,21 @@ def belonging_to_non_existent_class(model_name, assoc_name, options = {})
       expect(Parent.new).to have_many(:children)
     end
 
+    it 'rejects a :through association where there is no belongs_to in the inverse model' do
+      define_model :Child
+      define_model :Conception, child_id: :integer, parent_id: :integer
+      parent_class = define_model :Parent do
+        has_many :conceptions
+        has_many :children, through: :conceptions
+      end
+
+      expect { have_many(:children) }
+        .not_to match_against(parent_class.new)
+        .and_fail_with(<<-MESSAGE)
+Expected Parent to have a has_many association called children through conceptions (Could not find the source association(s) "child" or :children in model Conception. Try 'has_many :children, :through => :conceptions, :source => <name>'. Is it one of ?)
+        MESSAGE
+    end
+
     it 'accepts a valid association with an :as option' do
       define_model :child, guardian_type: :string, guardian_id: :integer
       define_model :parent do