From 6ca44086115d36d2d7ec490d51e1512d890b95f7 Mon Sep 17 00:00:00 2001
From: fatkodima <fatkodima123@gmail.com>
Date: Thu, 21 Dec 2023 02:14:59 +0200
Subject: [PATCH] Fix `find_by` for finding by associations with composite
 primary keys

---
 activerecord/lib/active_record/core.rb | 29 ++++++++++++++++++++++----
 activerecord/test/cases/finder_test.rb | 18 +++++++++++++++-
 2 files changed, 42 insertions(+), 5 deletions(-)

diff --git a/activerecord/lib/active_record/core.rb b/activerecord/lib/active_record/core.rb
index 87e56b00fb3cf..625dd365513fb 100644
--- a/activerecord/lib/active_record/core.rb
+++ b/activerecord/lib/active_record/core.rb
@@ -275,10 +275,25 @@ def find_by(*args) # :nodoc:
           elsif reflection.belongs_to? && !reflection.polymorphic?
             key = reflection.join_foreign_key
             pkey = reflection.join_primary_key
-            value = value.public_send(pkey) if value.respond_to?(pkey)
+
+            if pkey.is_a?(Array)
+              if pkey.all? { |attribute| value.respond_to?(attribute) }
+                value = pkey.map do |attribute|
+                  if attribute == "id"
+                    value.id_value
+                  else
+                    value.public_send(attribute)
+                  end
+                end
+                composite_primary_key = true
+              end
+            else
+              value = value.public_send(pkey) if value.respond_to?(pkey)
+            end
           end
 
-          if !columns_hash.key?(key) || StatementCache.unsupported_value?(value)
+          if !composite_primary_key &&
+            (!columns_hash.key?(key) || StatementCache.unsupported_value?(value))
             return super
           end
 
@@ -405,12 +420,18 @@ def table_metadata
 
         def cached_find_by(keys, values)
           statement = cached_find_by_statement(keys) { |params|
-            wheres = keys.index_with { params.bind }
+            wheres = keys.index_with do |key|
+              if key.is_a?(Array)
+                [key.map { params.bind }]
+              else
+                params.bind
+              end
+            end
             where(wheres).limit(1)
           }
 
           begin
-            statement.execute(values, connection).first
+            statement.execute(values.flatten, connection).first
           rescue TypeError
             raise ActiveRecord::StatementInvalid
           end
diff --git a/activerecord/test/cases/finder_test.rb b/activerecord/test/cases/finder_test.rb
index 66be08220e452..5b55bb77d4346 100644
--- a/activerecord/test/cases/finder_test.rb
+++ b/activerecord/test/cases/finder_test.rb
@@ -34,7 +34,7 @@ class FinderTest < ActiveRecord::TestCase
 
   fixtures :companies, :topics, :entrants, :developers, :developers_projects,
     :posts, :comments, :accounts, :authors, :author_addresses, :customers,
-    :categories, :categorizations, :cars, :clothing_items, :cpk_books
+    :categories, :categorizations, :cars, :clothing_items, :cpk_books, :cpk_reviews
 
   def test_find_by_id_with_hash
     assert_nothing_raised do
@@ -1875,6 +1875,22 @@ def test_finder_with_offset_string
     assert_equal books.map(&:id), Cpk::Book.order(author_id: :asc).find(books.map(&:id)).map(&:id)
   end
 
+  test "#find_by with composite primary key" do
+    book = cpk_books(:cpk_book_with_generated_pk)
+    assert_equal cpk_reviews(:first_book_review), Cpk::Review.find_by(book: book)
+  end
+
+  test "#find_by with composite primary key and query caching" do
+    book = cpk_books(:cpk_book_with_generated_pk)
+
+    Cpk::Review.cache do
+      assert_queries_count(1) do
+        Cpk::Review.find_by(book: book)
+        Cpk::Review.find_by(book: book)
+      end
+    end
+  end
+
   private
     def table_with_custom_primary_key
       yield(Class.new(Toy) do