Join table check for habtm w/ custom foreign keys should still work

If the habtm association in question specifies a custom foreign_key or
association_foreign_key, when the habtm matcher checks that the join
table has the correct columns, it should use the specified column names
instead of the defaults.

lib/shoulda/matchers/active_record/association_matcher.rb

@@ -957,7 +957,8 @@ module Shoulda
         end
 
         def join_table_matcher
-          @join_table_matcher ||= AssociationMatchers::JoinTableMatcher.new(self)
+          @join_table_matcher ||=
+            AssociationMatchers::JoinTableMatcher.new(self, reflector)
         end
 
         def class_exists?

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

@@ -4,7 +4,8 @@ module Shoulda
       module AssociationMatchers
         # @private
         class JoinTableMatcher
-          attr_reader :association_matcher, :failure_message
+          attr_reader :failure_message
+
           alias :missing_option :failure_message
 
           delegate :model_class, :join_table, :associated_class,
@@ -12,8 +13,9 @@ module Shoulda
 
           delegate :connection, to: :model_class
 
-          def initialize(association_matcher)
+          def initialize(association_matcher, reflector)
             @association_matcher = association_matcher
+            @reflector = reflector
           end
 
           def matches?(subject)
@@ -39,8 +41,14 @@ module Shoulda
             end
           end
 
+          protected
+
+          attr_reader :association_matcher, :reflector
+
           private
 
+          delegate :foreign_key, :association_foreign_key, to: :reflector
+
           def missing_columns
             @missing_columns ||= expected_join_table_columns.select do |key|
               !actual_join_table_columns.include?(key)
@@ -48,10 +56,7 @@ module Shoulda
           end
 
           def expected_join_table_columns
-            [
-              "#{model_class.name.underscore}_id",
-              "#{associated_class.name.underscore}_id"
-            ]
+            [foreign_key, association_foreign_key]
           end
 
           def actual_join_table_columns

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

@@ -45,6 +45,25 @@ module Shoulda
             end
           end
 
+          def foreign_key
+            if has_and_belongs_to_many_reflection
+              has_and_belongs_to_many_reflection.foreign_key
+            elsif reflection.respond_to?(:primary_key_name)
+              reflection.primary_key_name
+            else
+              reflection.foreign_key
+            end
+          end
+
+          def association_foreign_key
+            if has_and_belongs_to_many_reflection
+              join_model = has_and_belongs_to_many_reflection.options[:class]
+              join_model.right_reflection.foreign_key
+            else
+              reflection.association_foreign_key
+            end
+          end
+
           protected
 
           attr_reader :reflection, :subject

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

@@ -5,7 +5,8 @@ module Shoulda
         # @private
         class ModelReflector
           delegate :associated_class, :through?, :join_table,
-            :association_relation, :polymorphic?, to: :reflection
+            :association_relation, :polymorphic?, :foreign_key,
+            :association_foreign_key, to: :reflection
 
           def initialize(subject, name)
             @subject = subject

spec/shoulda/matchers/active_record/association_matcher_spec.rb

@@ -732,6 +732,44 @@ describe Shoulda::Matchers::ActiveRecord::AssociationMatcher do
       end.to fail_with_message_including('missing columns: person_id, relative_id')
     end
 
+    context 'using a custom foreign key' do
+      it 'rejects an association with a join table with incorrect columns' do
+        define_model :relative
+        define_model :person do
+          has_and_belongs_to_many :relatives,
+            foreign_key: :custom_foreign_key_id
+        end
+
+        define_model :people_relative,
+          id: false,
+          custom_foreign_key_id: :integer,
+          some_crazy_id: :integer
+
+        expect do
+          expect(Person.new).to have_and_belong_to_many(:relatives)
+        end.to fail_with_message_including('missing columns: custom_foreign_key_id, relative_id')
+      end
+    end
+
+    context 'using a custom association foreign key' do
+      it 'rejects an association with a join table with incorrect columns' do
+        define_model :relative
+        define_model :person do
+          has_and_belongs_to_many :relatives,
+            association_foreign_key: :custom_association_foreign_key_id
+        end
+
+        define_model :people_relative,
+          id: false,
+          custom_association_foreign_key_id: :integer,
+          some_crazy_id: :integer
+
+        expect do
+          expect(Person.new).to have_and_belong_to_many(:relatives)
+        end.to fail_with_message_including('missing columns: person_id, custom_association_foreign_key_id')
+      end
+    end
+
     it 'rejects an association of the wrong type' do
       define_model :relative, person_id: :integer
       define_model :person do