Don't error when `dup`ing a record with no PK

activerecord/lib/active_record/attribute_set.rb

@@ -52,6 +52,12 @@ def initialize_clone(_)
       super
     end
 
+    def reset(key)
+      if include?(key)
+        write_from_database(key, nil)
+      end
+    end
+
     protected
 
     attr_reader :attributes

activerecord/lib/active_record/core.rb

@@ -315,9 +315,8 @@ def init_with(coder)
 
     ##
     def initialize_dup(other) # :nodoc:
-      pk = self.class.primary_key
       @attributes = @attributes.dup
-      @attributes.write_from_database(pk, nil)
+      @attributes.reset(self.class.primary_key)
 
       run_callbacks(:initialize) unless _initialize_callbacks.empty?
 

activerecord/test/cases/dup_test.rb

@@ -141,5 +141,17 @@ def test_dup_with_default_scope
     ensure
       Topic.default_scopes = prev_default_scopes
     end
+
+    def test_dup_without_primary_key
+      klass = Class.new(ActiveRecord::Base) do
+        self.table_name = 'parrots_pirates'
+      end
+
+      record = klass.create!
+
+      assert_nothing_raised do
+        record.dup
+      end
+    end
   end
 end