Read has_one through association target from memory for new records.

When reading an `has_one` through association for a new record and loading from DB
is not possible retrieves the association target from the in memory through association.

Fixes #42387

activerecord/CHANGELOG.md

@@ -1,3 +1,30 @@
+*   Read `has_one` through association target from memory for new records.
+
+    When reading an `has_one` through association for a new record and loading from DB
+    is not possible retrieves the association target from the in memory through association:
+
+    ```ruby
+    class Book < ActiveRecord::Base
+      belongs_to :author
+    end
+
+    class Author < ActiveRecord::Base
+    end
+
+    class Reader < ActiveRecord::Base
+      has_one :book
+      has_one :author, through: :book
+    end
+
+    author = Author.new
+    book = Book.new(author: author)
+    reader = Reader.new(book: book)
+    reader.author
+    # => correctly returns `author`, previously returns `nil`.
+    ```
+
+    *Jacopo Beschi*
+
 *   Allow preloading of associations with instance dependent scopes
 
     *John Hawthorn*, *John Crepezzi*, *Adam Hess*, *Eileen M. Uchitelle*, *Dinah Shi*

activerecord/lib/active_record/associations/association.rb

@@ -176,7 +176,7 @@ def extensions
       # ActiveRecord::RecordNotFound is rescued within the method, and it is
       # not reraised. The proxy is \reset and +nil+ is the return value.
       def load_target
-        @target = find_target if (@stale_state && stale_target?) || find_target?
+        @target = find_target if load_target?
 
         loaded! unless loaded?
         target
@@ -277,6 +277,10 @@ def scope_for_create
           scope.scope_for_create
         end
 
+        def load_target?
+          (@stale_state && stale_target?) || find_target?
+        end
+
         def find_target?
           !loaded? && (!owner.new_record? || foreign_key_present?) && klass
         end

activerecord/lib/active_record/associations/has_one_through_association.rb

@@ -6,6 +6,14 @@ module Associations
     class HasOneThroughAssociation < HasOneAssociation #:nodoc:
       include ThroughAssociation
 
+      def reader
+        if load_target_from_memory?
+          self.target = find_target_from_memory
+        else
+          super
+        end
+      end
+
       private
         def replace(record, save = true)
           create_through_record(record, save)
@@ -40,6 +48,16 @@ def create_through_record(record, save)
             end
           end
         end
+
+        def load_target_from_memory?
+          !load_target? && !loaded? && owner.new_record?
+        end
+
+        # Recursively reads the through_associations and collects their values.
+        def find_target_from_memory
+          through_record = owner.association(reflection.through_reflection.name).reader
+          through_record&.association(source_reflection.name)&.reader
+        end
     end
   end
 end

activerecord/test/cases/associations/has_one_through_associations_test.rb

@@ -445,4 +445,14 @@ def test_has_one_through_do_not_cache_association_reader_if_the_though_method_ha
   ensure
     CustomerCarrier.current_customer = nil
   end
+
+  def test_has_one_through_loads_target_from_memory
+    category = Category.new
+    club = Club.new(name: "LRUG", category: category)
+    current_membership = CurrentMembership.new(club: club)
+    new_member = Member.new(name: "Chris", current_membership: current_membership)
+
+    assert_equal club, new_member.club
+    assert_equal category, new_member.club_category
+  end
 end