diff --git a/lib/ar2dto/model_config.rb b/lib/ar2dto/model_config.rb
index 4672a0a..a084d73 100644
--- a/lib/ar2dto/model_config.rb
+++ b/lib/ar2dto/model_config.rb
@@ -13,26 +13,25 @@ def setup_config(configs)
     end
 
     def except
-      configs["except"]
+      @except ||= configs["except"]
     end
 
     def class_name
-      configs["class_name"] ||
-        model_name.sub(/#{configs["replace_suffix"]["from"]}$/, configs["replace_suffix"]["to"].to_s)
+      @class_name ||= namespaced_class_name.split("::").last
     end
 
     def namespace
-      @namespace ||= model.name.deconstantize.presence&.constantize || Object
+      @namespace ||= namespaced_class_name.deconstantize.presence&.constantize || Object
     end
 
     def namespaced_class_name
-      "#{namespace}::#{class_name}"
+      @namespaced_class_name ||= configs["class_name"] || model_name_replaced_suffix
     end
 
     private
 
-    def model_name
-      model.name.split("::").last
+    def model_name_replaced_suffix
+      model.name.sub(/#{configs["replace_suffix"]["from"]}$/, configs["replace_suffix"]["to"].to_s)
     end
   end
 end
diff --git a/spec/ar2dto/options_spec.rb b/spec/ar2dto/options_spec.rb
index c62c521..b319c97 100644
--- a/spec/ar2dto/options_spec.rb
+++ b/spec/ar2dto/options_spec.rb
@@ -31,6 +31,12 @@ def self.name
     it "respects the custom class_name" do
       expect(Car.new.to_dto).to be_a(VehicleDTO)
     end
+
+    context "with a namespace" do
+      it "respects the custom class_name" do
+        expect(Shop::LineItem.new.to_dto).to be_a(Core::LineItemDTO)
+      end
+    end
   end
 
   describe "option replace_suffix" do
diff --git a/spec/ar2dto/record_to_dto/namespaces_spec.rb b/spec/ar2dto/record_to_dto/namespaces_spec.rb
index fb50f5c..9660a38 100644
--- a/spec/ar2dto/record_to_dto/namespaces_spec.rb
+++ b/spec/ar2dto/record_to_dto/namespaces_spec.rb
@@ -2,19 +2,30 @@
 
 RSpec.describe "#to_dto" do
   describe "namespaces" do
-    context "with a namespaced model" do
+    let(:attributes) do
+      {
+        user_id: 1
+      }
+    end
+
+    subject { order.to_dto }
+
+    let(:order) { Shop::Order.new(attributes) }
+
+    it { is_expected.to be_a(Shop::OrderDTO) }
+
+    context "with a custom class_name" do
       let(:attributes) do
         {
-          user_id: 1
+          order_id: 1
         }
       end
-      let(:options) { {} }
 
-      subject { order.to_dto(options) }
+      subject { line_item.to_dto }
 
-      let(:order) { Shop::Order.new(attributes) }
+      let(:line_item) { Shop::LineItem.new(attributes) }
 
-      it { is_expected.to be_a(Shop::OrderDTO) }
+      it { is_expected.to be_a(Core::LineItemDTO) }
     end
   end
 end
diff --git a/spec/support/fixtures/core.rb b/spec/support/fixtures/core.rb
new file mode 100644
index 0000000..87cb4ca
--- /dev/null
+++ b/spec/support/fixtures/core.rb
@@ -0,0 +1,4 @@
+# frozen_string_literal: true
+
+module Core
+end
diff --git a/spec/support/fixtures/shop/line_item.rb b/spec/support/fixtures/shop/line_item.rb
new file mode 100644
index 0000000..30c668a
--- /dev/null
+++ b/spec/support/fixtures/shop/line_item.rb
@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+module Shop
+  class LineItem < ActiveRecord::Base
+    self.table_name = "shop_line_items"
+
+    belongs_to :order
+
+    has_dto class_name: "Core::LineItemDTO"
+  end
+end
diff --git a/spec/support/schema.rb b/spec/support/schema.rb
index e8eba9c..5d2f7fa 100644
--- a/spec/support/schema.rb
+++ b/spec/support/schema.rb
@@ -33,4 +33,12 @@
 
     t.timestamps
   end
+
+  create_table :shop_line_items, force: true do |t|
+    t.bigint :order_id
+    t.string :name
+    t.bigint :price
+
+    t.timestamps
+  end
 end