Fix find_by with custom primary key for belongs_to association

Similar to rails#40809 which is fixed by rails#40815, but it is caused by dbda5fe.

`value.id if value.respond_to?(:id)` is not always correct if primary
key is customized on the belongs_to association.

Use `reflection.join_primary_key` in that case.

Fixes rails#41081.

Author: kamipo

activerecord/lib/active_record/core.rb

@@ -332,31 +332,37 @@ def find_by(*args) # :nodoc:
         hash = args.first
         return super unless Hash === hash
 
-        values = hash.values.map! { |value| value.respond_to?(:id) ? value.id : value }
-        return super if values.any? { |v| StatementCache.unsupported_value?(v) }
-
-        keys = hash.keys.map! do |key|
-          attribute_aliases[name = key.to_s] || begin
-            reflection = _reflect_on_association(name)
-            if reflection&.belongs_to? && !reflection.polymorphic?
-              reflection.join_foreign_key
-            elsif reflect_on_aggregation(name)
-              return super
-            else
-              name
-            end
+        hash = hash.each_with_object({}) do |(key, value), h|
+          key = key.to_s
+          key = attribute_aliases[key] || key
+
+          return super if reflect_on_aggregation(key)
+
+          reflection = _reflect_on_association(key)
+
+          if !reflection
+            value = value.id if value.respond_to?(:id)
+          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)
+          end
+
+          if !columns_hash.key?(key) || StatementCache.unsupported_value?(value)
+            return super
           end
-        end
 
-        return super unless keys.all? { |k| columns_hash.key?(k) }
+          h[key] = value
+        end
 
+        keys = hash.keys
         statement = cached_find_by_statement(keys) { |params|
           wheres = keys.index_with { params.bind }
           where(wheres).limit(1)
         }
 
         begin
-          statement.execute(values, connection).first
+          statement.execute(hash.values, connection).first
         rescue TypeError
           raise ActiveRecord::StatementInvalid
         end

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -46,6 +46,10 @@ def test_where_with_custom_primary_key
     assert_equal [authors(:david)], Author.where(owned_essay: essays(:david_modest_proposal))
   end
 
+  def test_find_by_with_custom_primary_key
+    assert_equal authors(:david), Author.find_by(owned_essay: essays(:david_modest_proposal))
+  end
+
   def test_where_on_polymorphic_association_with_nil
     assert_equal comments(:greetings), Comment.where(author: nil).first
     assert_equal comments(:greetings), Comment.where(author: [nil]).first