[BAC-1267] Compile the JSON attribute assigner per exact class

Gemfile.lock

@@ -1,7 +1,7 @@
 PATH
   remote: .
   specs:
-    activemodel-entity (0.8.0)
+    activemodel-entity (0.13.0)
       actionpack (>= 7)
       activemodel (>= 7)
       activesupport (>= 7)
@@ -69,19 +69,23 @@ GEM
     loofah (2.22.0)
       crass (~> 1.0.2)
       nokogiri (>= 1.12.0)
-    minitest (5.24.1)
+    mini_portile2 (2.8.9)
+    minitest (6.0.6)
+      drb (~> 2.0)
+      prism (~> 1.5)
     mutex_m (0.2.0)
-    nokogiri (1.16.6-aarch64-linux)
+    nokogiri (1.19.4)
+      mini_portile2 (~> 2.8.2)
       racc (~> 1.4)
-    nokogiri (1.16.6-arm-linux)
+    nokogiri (1.19.4-aarch64-linux-gnu)
       racc (~> 1.4)
-    nokogiri (1.16.6-arm64-darwin)
+    nokogiri (1.19.4-arm-linux-gnu)
       racc (~> 1.4)
-    nokogiri (1.16.6-x86-linux)
+    nokogiri (1.19.4-arm64-darwin)
       racc (~> 1.4)
-    nokogiri (1.16.6-x86_64-darwin)
+    nokogiri (1.19.4-x86_64-darwin)
       racc (~> 1.4)
-    nokogiri (1.16.6-x86_64-linux)
+    nokogiri (1.19.4-x86_64-linux-gnu)
       racc (~> 1.4)
     parallel (1.25.1)
     parser (3.3.4.0)
@@ -90,6 +94,7 @@ GEM
     pp (0.6.2)
       prettyprint
     prettyprint (0.2.0)
+    prism (1.9.0)
     psych (5.2.6)
       date
       stringio

lib/active_model/entity/parsers/json.rb

@@ -11,45 +11,49 @@ def set_attribute_from_json(name, value)
           @attributes[name] = @attributes[name].with_value_from_json(value)
         end
 
+        def assign_attributes_from_json(json)
+          instance_exec(json, &self.class.json_attributes_assigner)
+        end
+
         # Class-level methods.
         module ClassMethods
           def from_json(json)
             new.tap do |instance|
               instance.assign_attributes_from_json(json)
             end
           end
-        end
 
-        def method_missing(method_name, *, &)
-          if method_name == :assign_attributes_from_json
-            setters = attributes.keys.map do |name|
+          # Compiled once per exact class (class-level ivars are not
+          # inherited), so an assigner compiled for a superclass can never
+          # shadow attributes that exist only on a subclass.
+          def json_attributes_assigner
+            @json_attributes_assigner ||= compile_json_attributes_assigner
+          end
+
+          private
+
+          def compile_json_attributes_assigner
+            setters = attribute_types.keys.map do |name|
               <<~RUBY
                 #{name}_value = json[#{name.camelize(:lower).inspect}]
                 #{name}_value = json[#{name.camelize(:lower).to_sym.inspect}] if #{name}_value.nil?
 
-                self.set_attribute_from_json(
+                set_attribute_from_json(
                   #{name.inspect},
                   #{name}_value
                 )
               RUBY
             end
 
             code = <<~RUBY
-              def assign_attributes_from_json(json)
+              ->(json) do
                 #{setters.join("\n")}
               end
             RUBY
 
-            self.class.class_eval(code)
-            send(method_name, *, &)
-          else
-            super
+            class_eval(code, __FILE__, __LINE__)
           end
         end
-
-        def respond_to_missing?(method_name, include_private)
-          method_name == :assign_attributes_from_json || super
-        end
       end
     end
   end

spec/active_model/entity/parsers/json_spec.rb

@@ -17,6 +17,16 @@ class Role
     attribute :field_positions, :array, of: "ParsersTest::Position"
   end
 
+  class Employee
+    include ActiveModel::Entity
+
+    attribute :field_name, :string
+  end
+
+  class Manager < Employee
+    attribute :field_department, :string
+  end
+
   class Person
     include ActiveModel::Entity
 
@@ -121,6 +131,17 @@ class Person
     end
   end
 
+  context "parsing into a subclass after the superclass has compiled its assigner" do
+    it "keeps subclass-only attributes" do
+      employee = ParsersTest::Employee.from_json({ "fieldName" => "emp" })
+      manager = ParsersTest::Manager.from_json({ "fieldName" => "mgr", "fieldDepartment" => "sales" })
+
+      expect(employee.field_name).to eq("emp")
+      expect(manager.field_name).to eq("mgr")
+      expect(manager.field_department).to eq("sales")
+    end
+  end
+
   context "parsing symbolized json" do
     let(:source) do
       {