Remove useless preloader classes

They are only different by one line of code which doesn't deserve a
hierarchy of 7 classes.

Closes #31079.

[Ryuta Kamizono & Bogdan Gusiev]

9 files changed, 8 insertions, 85 deletions

diff --git a/activerecord/lib/active_record/associations/preloader/association.rb b/activerecord/lib/active_record/associations/preloader/association.rb
index 19c337dc39..735da152b7 100644
--- a/activerecord/lib/active_record/associations/preloader/association.rb
+++ b/activerecord/lib/active_record/associations/preloader/association.rb
@@ -42,7 +42,13 @@ module ActiveRecord
           end
 
           def associate_records_to_owner(owner, records)
-            raise NotImplementedError
+            association = owner.association(reflection.name)
+            if reflection.collection?
+              association.loaded!
+              association.target.concat(records)
+            else
+              association.target = records.first
+            end
           end
 
           def owner_keys
diff --git a/activerecord/lib/active_record/associations/preloader/belongs_to.rb b/activerecord/lib/active_record/associations/preloader/belongs_to.rb
deleted file mode 100644
index a8e3340b23..0000000000
--- a/activerecord/lib/active_record/associations/preloader/belongs_to.rb
+++ /dev/null
@@ -1,10 +0,0 @@
-# frozen_string_literal: true
-
-module ActiveRecord
-  module Associations
-    class Preloader
-      class BelongsTo < SingularAssociation #:nodoc:
-      end
-    end
-  end
-end
diff --git a/activerecord/lib/active_record/associations/preloader/collection_association.rb b/activerecord/lib/active_record/associations/preloader/collection_association.rb
deleted file mode 100644
index fc2029f54a..0000000000
--- a/activerecord/lib/active_record/associations/preloader/collection_association.rb
+++ /dev/null
@@ -1,16 +0,0 @@
-# frozen_string_literal: true
-
-module ActiveRecord
-  module Associations
-    class Preloader
-      class CollectionAssociation < Association #:nodoc:
-        private
-          def associate_records_to_owner(owner, records)
-            association = owner.association(reflection.name)
-            association.loaded!
-            association.target.concat(records)
-          end
-      end
-    end
-  end
-end
diff --git a/activerecord/lib/active_record/associations/preloader/has_many.rb b/activerecord/lib/active_record/associations/preloader/has_many.rb
deleted file mode 100644
index 72f55bc43f..0000000000
--- a/activerecord/lib/active_record/associations/preloader/has_many.rb
+++ /dev/null
@@ -1,10 +0,0 @@
-# frozen_string_literal: true
-
-module ActiveRecord
-  module Associations
-    class Preloader
-      class HasMany < CollectionAssociation #:nodoc:
-      end
-    end
-  end
-end
diff --git a/activerecord/lib/active_record/associations/preloader/has_many_through.rb b/activerecord/lib/active_record/associations/preloader/has_many_through.rb
deleted file mode 100644
index 3e17d07a33..0000000000
--- a/activerecord/lib/active_record/associations/preloader/has_many_through.rb
+++ /dev/null
@@ -1,11 +0,0 @@
-# frozen_string_literal: true
-
-module ActiveRecord
-  module Associations
-    class Preloader
-      class HasManyThrough < CollectionAssociation #:nodoc:
-        include ThroughAssociation
-      end
-    end
-  end
-end
diff --git a/activerecord/lib/active_record/associations/preloader/has_one.rb b/activerecord/lib/active_record/associations/preloader/has_one.rb
deleted file mode 100644
index e339b65fb5..0000000000
--- a/activerecord/lib/active_record/associations/preloader/has_one.rb
+++ /dev/null
@@ -1,10 +0,0 @@
-# frozen_string_literal: true
-
-module ActiveRecord
-  module Associations
-    class Preloader
-      class HasOne < SingularAssociation #:nodoc:
-      end
-    end
-  end
-end
diff --git a/activerecord/lib/active_record/associations/preloader/has_one_through.rb b/activerecord/lib/active_record/associations/preloader/has_one_through.rb
deleted file mode 100644
index 17734d0257..0000000000
--- a/activerecord/lib/active_record/associations/preloader/has_one_through.rb
+++ /dev/null
@@ -1,11 +0,0 @@
-# frozen_string_literal: true
-
-module ActiveRecord
-  module Associations
-    class Preloader
-      class HasOneThrough < SingularAssociation #:nodoc:
-        include ThroughAssociation
-      end
-    end
-  end
-end
diff --git a/activerecord/lib/active_record/associations/preloader/singular_association.rb b/activerecord/lib/active_record/associations/preloader/singular_association.rb
deleted file mode 100644
index 30a92411e3..0000000000
--- a/activerecord/lib/active_record/associations/preloader/singular_association.rb
+++ /dev/null
@@ -1,15 +0,0 @@
-# frozen_string_literal: true
-
-module ActiveRecord
-  module Associations
-    class Preloader
-      class SingularAssociation < Association #:nodoc:
-        private
-          def associate_records_to_owner(owner, records)
-            association = owner.association(reflection.name)
-            association.target = records.first
-          end
-      end
-    end
-  end
-end
diff --git a/activerecord/lib/active_record/associations/preloader/through_association.rb b/activerecord/lib/active_record/associations/preloader/through_association.rb
index 762275fbad..a6b7ab80a2 100644
--- a/activerecord/lib/active_record/associations/preloader/through_association.rb
+++ b/activerecord/lib/active_record/associations/preloader/through_association.rb
@@ -3,7 +3,7 @@
 module ActiveRecord
   module Associations
     class Preloader
-      module ThroughAssociation #:nodoc:
+      class ThroughAssociation < Association # :nodoc:
         def run(preloader)
           already_loaded     = owners.first.association(through_reflection.name).loaded?
           through_scope      = through_scope()