Refactor Preloader to remove AlreadyLoaded class

Part 2 of ?

It felt strange to have an `AlreadyLoaded` class when we have 2 classes
for the preloaders. `ThroughAssociation` already super's to
`Association`. Instead of having a new class that provides a confusing
indirection, we can get and set all the information we need on
`Association` inside of `run`.

In addition, the `already_loaded` instance variable on
`ThroughAssociation` actually meant something different than
`already_loaded` that was checked for `AlreadyLoaded`. By pushing this
check down we can get rid of the `initialize` in `ThroughAssociation`
entirely since then all it's doing is calling `super`.

Co-authored-by: John Crepezzi <john.crepezzi@gmail.com>

activerecord/lib/active_record/associations/preloader.rb

@@ -135,7 +135,7 @@ def build_child_preloader(reflection, child, loaders)
 
         def preloaders_for_reflection(reflection, reflection_records)
           reflection_records.group_by { |record| record.association(reflection.name).klass }.map do |rhs_klass, rs|
-            preloader_for(reflection, rs).new(rhs_klass, rs, reflection, scope, associate_by_default).run
+            preloader_for(reflection).new(rhs_klass, rs, reflection, scope, associate_by_default).run
           end
         end
 
@@ -149,37 +149,10 @@ def grouped_records(association)
           h
         end
 
-        class AlreadyLoaded # :nodoc:
-          def initialize(klass, owners, reflection, preload_scope, associate_by_default = true)
-            @owners = owners
-            @reflection = reflection
-          end
-
-          def run
-            self
-          end
-
-          def preloaded_records
-            @preloaded_records ||= records_by_owner.flat_map(&:last)
-          end
-
-          def records_by_owner
-            @records_by_owner ||= owners.index_with do |owner|
-              Array(owner.association(reflection.name).target)
-            end
-          end
-
-          private
-            attr_reader :owners, :reflection
-        end
-
         # Returns a class containing the logic needed to load preload the data
         # and attach it to a relation. The class returned implements a `run` method
         # that accepts a preloader.
-        def preloader_for(reflection, owners)
-          if owners.all? { |o| o.association(reflection.name).loaded? }
-            return AlreadyLoaded
-          end
+        def preloader_for(reflection)
           reflection.check_preloadable!
 
           if reflection.options[:through]

activerecord/lib/active_record/associations/preloader/association.rb

@@ -11,9 +11,16 @@ def initialize(klass, owners, reflection, preload_scope, associate_by_default =
           @preload_scope = preload_scope
           @associate     = associate_by_default || !preload_scope || preload_scope.empty_scope?
           @model         = owners.first && owners.first.class
+
+          @already_loaded = owners.all? { |o| o.association(reflection.name).loaded? }
         end
 
         def run
+          if @already_loaded
+            fetch_from_preloaded_records
+            return self
+          end
+
           records = records_by_owner
 
           owners.each do |owner|
@@ -38,6 +45,14 @@ def preloaded_records
         private
           attr_reader :owners, :reflection, :preload_scope, :model, :klass
 
+          def fetch_from_preloaded_records
+            @records_by_owner = owners.index_with do |owner|
+              Array(owner.association(reflection.name).target)
+            end
+
+            @preloaded_records = records_by_owner.flat_map(&:last)
+          end
+
           def load_records
             # owners can be duplicated when a relation has a collection association join
             # #compare_by_identity makes such owners different hash keys

activerecord/lib/active_record/associations/preloader/through_association.rb

@@ -4,11 +4,6 @@ module ActiveRecord
   module Associations
     class Preloader
       class ThroughAssociation < Association # :nodoc:
-        def initialize(*)
-          super
-          @already_loaded = owners.first.association(through_reflection.name).loaded?
-        end
-
         def preloaded_records
           @preloaded_records ||= source_preloaders.flat_map(&:preloaded_records)
         end
@@ -21,7 +16,7 @@ def records_by_owner
           @records_by_owner = owners.each_with_object({}) do |owner, result|
             through_records = through_records_by_owner[owner] || []
 
-            if @already_loaded
+            if owners.first.association(through_reflection.name).loaded?
               if source_type = reflection.options[:source_type]
                 through_records = through_records.select do |record|
                   record[reflection.foreign_type] == source_type