DEV: Add support for Rails 6

Minor fixes to add Rails 6 support to Discourse, we now will boot
with RAILS_MASTER=1, all specs pass

Only one tiny deprecation left

Largest change was the way ActiveModel:Errors changed interface a
bit but there is a simple backwards compat way of working it

spec/components/concern/has_custom_fields_spec.rb

@@ -22,10 +22,10 @@ describe HasCustomFields do
       DB.exec("drop table custom_fields_test_items")
       DB.exec("drop table custom_fields_test_item_custom_fields")
 
-      # import is making my life hard, we need to nuke this out of orbit
-      des = ActiveSupport::DescendantsTracker.class_variable_get :@@direct_descendants
-      des[ActiveRecord::Base].delete(CustomFieldsTestItem)
-      des[ActiveRecord::Base].delete(CustomFieldsTestItemCustomField)
+      # this weakref in the descendant tracker should clean up the two tests
+      # if this becomes an issue we can revisit (watch out for erratic tests)
+      Object.send(:remove_const, :CustomFieldsTestItem)
+      Object.send(:remove_const, :CustomFieldsTestItemCustomField)
     end
 
     it "simple modification of custom fields" do

spec/components/concern/has_search_data_spec.rb

@@ -21,10 +21,10 @@ describe HasSearchData do
       DB.exec("drop table model_items")
       DB.exec("drop table model_item_search_data")
 
-      # import is making my life hard, we need to nuke this out of orbit
-      des = ActiveSupport::DescendantsTracker.class_variable_get :@@direct_descendants
-      des[ActiveRecord::Base].delete(ModelItem)
-      des[ActiveRecord::Base].delete(ModelItemSearchData)
+      # this weakref in the descendant tracker should clean up the two tests
+      # if this becomes an issue we can revisit (watch out for erratic tests)
+      Object.send(:remove_const, :ModelItem)
+      Object.send(:remove_const, :ModelItemSearchData)
     end
 
     let(:item) do

spec/components/concern/positionable_spec.rb

@@ -20,9 +20,9 @@ describe Positionable do
     after do
       DB.exec("drop table test_items")
 
-      # import is making my life hard, we need to nuke this out of orbit
-      des = ActiveSupport::DescendantsTracker.class_variable_get :@@direct_descendants
-      des[ActiveRecord::Base].delete(TestItem)
+      # this weakref in the descendant tracker should clean up the two tests
+      # if this becomes an issue we can revisit (watch out for erratic tests)
+      Object.send(:remove_const, :TestItem)
     end
 
     it "can position stuff correctly" do

spec/components/concern/searchable_spec.rb

@@ -22,10 +22,10 @@ describe Searchable do
       DB.exec("drop table searchable_records")
       DB.exec("drop table searchable_record_search_data")
 
-      # import is making my life hard, we need to nuke this out of orbit
-      des = ActiveSupport::DescendantsTracker.class_variable_get :@@direct_descendants
-      des[ActiveRecord::Base].delete(SearchableRecord)
-      des[ActiveRecord::Base].delete(SearchableRecordSearchData)
+      # this weakref in the descendant tracker should clean up the two tests
+      # if this becomes an issue we can revisit (watch out for erratic tests)
+      Object.send(:remove_const, :SearchableRecord)
+      Object.send(:remove_const, :SearchableRecordSearchData)
     end
 
     let(:item) { SearchableRecord.create! }

spec/components/validators/post_validator_spec.rb

@@ -227,7 +227,7 @@ describe Validators::PostValidator do
 
       it "should add an error" do
         validator.unique_post_validator(new_post)
-        expect(new_post.errors.keys).to contain_exactly(:raw)
+        expect(new_post.errors.to_hash.keys).to contain_exactly(:raw)
       end
 
       it "should not add an error if post.skip_unique_check is true" do