Fixes polymorphic joins.

* Polymorphic joins have been missing a critical join constraint that
is responsible for only returning polymorphic records of the specified
type.

* Override ActiveRecord::Reflection::AbstractReflection#join_scope to
add this constraint if the reflection is polymorphic.

lib/polyamorous/activerecord_5.2_ruby_2/reflection.rb

@@ -2,10 +2,17 @@ module Polyamorous
   module ReflectionExtensions
     def build_join_constraint(table, foreign_table)
       if polymorphic?
-        super(table, foreign_table)
-        .and(foreign_table[foreign_type].eq(klass.name))
+        super.and(foreign_table[foreign_type].eq(klass.name))
       else
-        super(table, foreign_table)
+        super
+      end
+    end
+
+    def join_scope(table, foreign_table, foreign_klass)
+      if respond_to?(:polymorphic?) && polymorphic?
+        super.where!(foreign_table[foreign_type].eq(klass.name))
+      else
+        super
       end
     end
   end

spec/ransack/search_spec.rb

@@ -328,6 +328,7 @@ module Ransack
         s = Search.new(Note, notable_of_Person_type_name_eq: 'Ernie').result
         expect(s).to be_an ActiveRecord::Relation
         expect(s.to_sql).to match /#{people_name_field} = 'Ernie'/
+        expect(s.to_sql).to match /"notes"."notable_type" = 'Person'/
       end
 
       it 'evaluates nested conditions' do