Ensure AR #second, #third, etc. finders work through associations

This commit fixes two regressions introduced in cafe31a078 where
newly created finder methods #second, #third, #forth, and #fifth
caused a NoMethodError error on reload associations and where we
were pulling the wrong element out of cached associations.

Examples:

  some_book.authors.reload.second

  # Before
  # => NoMethodError: undefined method 'first' for nil:NilClass

  # After
  # => #<Author id: 2, name: "Sally Second", ...>

  some_book.first.authors.first
  some_book.first.authors.second

  # Before
  # => #<Author id: 1, name: "Freddy First", ...>
  # => #<Author id: 1, name: "Freddy First", ...>

  # After
  # => #<Author id: 1, name: "Freddy First", ...>
  # => #<Author id: 2, name: "Sally Second", ...>

Fixes #13783.

3 files changed, 59 insertions, 13 deletions

diff --git a/activerecord/lib/active_record/associations/collection_association.rb b/activerecord/lib/active_record/associations/collection_association.rb
index 52531a3520..89b7945c78 100644
--- a/activerecord/lib/active_record/associations/collection_association.rb
+++ b/activerecord/lib/active_record/associations/collection_association.rb
@@ -96,11 +96,31 @@ module ActiveRecord
       end
 
       def first(*args)
-        first_or_last(:first, *args)
+        first_nth_or_last(:first, *args)
+      end
+
+      def second(*args)
+        first_nth_or_last(:second, *args)
+      end
+
+      def third(*args)
+        first_nth_or_last(:third, *args)
+      end
+
+      def fourth(*args)
+        first_nth_or_last(:fourth, *args)
+      end
+
+      def fifth(*args)
+        first_nth_or_last(:fifth, *args)
+      end
+
+      def forty_two(*args)
+        first_nth_or_last(:forty_two, *args)
       end
 
       def last(*args)
-        first_or_last(:last, *args)
+        first_nth_or_last(:last, *args)
       end
 
       def build(attributes = {}, &block)
@@ -526,7 +546,7 @@ module ActiveRecord
         #   * target already loaded
         #   * owner is new record
         #   * target contains new or changed record(s)
-        def fetch_first_or_last_using_find?(args)
+        def fetch_first_nth_or_last_using_find?(args)
           if args.first.is_a?(Hash)
             true
           else
@@ -564,10 +584,10 @@ module ActiveRecord
         end
 
         # Fetches the first/last using SQL if possible, otherwise from the target array.
-        def first_or_last(type, *args)
+        def first_nth_or_last(type, *args)
           args.shift if args.first.is_a?(Hash) && args.first.empty?
 
-          collection = fetch_first_or_last_using_find?(args) ? scope : load_target
+          collection = fetch_first_nth_or_last_using_find?(args) ? scope : load_target
           collection.send(type, *args).tap do |record|
             set_inverse_instance record if record.is_a? ActiveRecord::Base
           end
diff --git a/activerecord/lib/active_record/associations/collection_proxy.rb b/activerecord/lib/active_record/associations/collection_proxy.rb
index 10434734e3..a67b834a80 100644
--- a/activerecord/lib/active_record/associations/collection_proxy.rb
+++ b/activerecord/lib/active_record/associations/collection_proxy.rb
@@ -170,6 +170,32 @@ module ActiveRecord
         @association.first(*args)
       end
 
+      # Same as +first+ except returns only the second record.
+      def second(*args)
+        @association.second(*args)
+      end
+
+      # Same as +first+ except returns only the third record.
+      def third(*args)
+        @association.third(*args)
+      end
+
+      # Same as +first+ except returns only the fourth record.
+      def fourth(*args)
+        @association.fourth(*args)
+      end
+
+      # Same as +first+ except returns only the fifth record.
+      def fifth(*args)
+        @association.fifth(*args)
+      end
+
+      # Same as +first+ except returns only the forty second record.
+      # Also known as accessing "the reddit".
+      def forty_two(*args)
+        @association.forty_two(*args)
+      end
+
       # Returns the last record, or the last +n+ records, from the collection.
       # If the collection is empty, the first form returns +nil+, and the second
       # form returns an empty array.
diff --git a/activerecord/lib/active_record/relation/finder_methods.rb b/activerecord/lib/active_record/relation/finder_methods.rb
index f2ac351a8b..2dd1e6f14b 100644
--- a/activerecord/lib/active_record/relation/finder_methods.rb
+++ b/activerecord/lib/active_record/relation/finder_methods.rb
@@ -129,7 +129,7 @@ module ActiveRecord
       if limit
         find_nth_with_limit(offset_value, limit)
       else
-        find_nth(offset_value)
+        find_nth(:first, offset_value)
       end
     end
 
@@ -179,7 +179,7 @@ module ActiveRecord
     #   Person.offset(3).second # returns the second object from OFFSET 3 (which is OFFSET 4)
     #   Person.where(["user_name = :u", { u: user_name }]).second
     def second
-      find_nth(offset_value ? offset_value + 1 : 1)
+      find_nth(:second, offset_value ? offset_value + 1 : 1)
     end
 
     # Same as +second+ but raises <tt>ActiveRecord::RecordNotFound</tt> if no record
@@ -195,7 +195,7 @@ module ActiveRecord
     #   Person.offset(3).third # returns the third object from OFFSET 3 (which is OFFSET 5)
     #   Person.where(["user_name = :u", { u: user_name }]).third
     def third
-      find_nth(offset_value ? offset_value + 2 : 2)
+      find_nth(:third, offset_value ? offset_value + 2 : 2)
     end
 
     # Same as +third+ but raises <tt>ActiveRecord::RecordNotFound</tt> if no record
@@ -211,7 +211,7 @@ module ActiveRecord
     #   Person.offset(3).fourth # returns the fourth object from OFFSET 3 (which is OFFSET 6)
     #   Person.where(["user_name = :u", { u: user_name }]).fourth
     def fourth
-      find_nth(offset_value ? offset_value + 3 : 3)
+      find_nth(:fourth, offset_value ? offset_value + 3 : 3)
     end
 
     # Same as +fourth+ but raises <tt>ActiveRecord::RecordNotFound</tt> if no record
@@ -227,7 +227,7 @@ module ActiveRecord
     #   Person.offset(3).fifth # returns the fifth object from OFFSET 3 (which is OFFSET 7)
     #   Person.where(["user_name = :u", { u: user_name }]).fifth
     def fifth
-      find_nth(offset_value ? offset_value + 4 : 4)
+      find_nth(:fifth, offset_value ? offset_value + 4 : 4)
     end
 
     # Same as +fifth+ but raises <tt>ActiveRecord::RecordNotFound</tt> if no record
@@ -243,7 +243,7 @@ module ActiveRecord
     #   Person.offset(3).forty_two # returns the fifth object from OFFSET 3 (which is OFFSET 44)
     #   Person.where(["user_name = :u", { u: user_name }]).forty_two
     def forty_two
-      find_nth(offset_value ? offset_value + 41 : 41)
+      find_nth(:forty_two, offset_value ? offset_value + 41 : 41)
     end
 
     # Same as +forty_two+ but raises <tt>ActiveRecord::RecordNotFound</tt> if no record
@@ -444,9 +444,9 @@ module ActiveRecord
       end
     end
 
-    def find_nth(offset)
+    def find_nth(ordinal, offset)
       if loaded?
-        @records.first
+        @records.send(ordinal)
       else
         @offsets[offset] ||= find_nth_with_limit(offset, 1).first
       end