Support nested attribute hashes rendered as hyphenated attributes

lib/arbre/html/attributes.rb

@@ -5,7 +5,7 @@ module HTML
     class Attributes < Hash
 
       def to_s
-        map do |name, value|
+        flatten_hash.map do |name, value|
           next if value_empty?(value)
           "#{html_escape(name)}=\"#{html_escape(value)}\""
         end.compact.join ' '
@@ -17,6 +17,18 @@ def any?
 
       protected
 
+      def flatten_hash(hash = self, old_path = [], accumulator = {})
+        hash.each do |key, value|
+          path = old_path + [key]
+          if value.is_a? Hash
+            flatten_hash(value, path, accumulator)
+          else
+            accumulator[path.join('-')] = value
+          end
+        end
+        accumulator
+      end
+
       def value_empty?(value)
         value.respond_to?(:empty?) ? value.empty? : !value
       end

spec/arbre/unit/html/tag_attributes_spec.rb

@@ -13,16 +13,50 @@
       expect(tag.attributes).to eq({id: "my_id"})
     end
 
-    it "should render the attributes to html" do
-      expect(tag.to_s).to eq "<tag id=\"my_id\"></tag>\n"
-    end
+    describe "#to_s" do
+      it "should render the attributes to html" do
+        expect(tag.to_s).to eq "<tag id=\"my_id\"></tag>\n"
+      end
+
+      it "shouldn't render attributes that are empty" do
+        tag.class_list # initializes an empty ClassList
+        tag.set_attribute :foo, ''
+        tag.set_attribute :bar, nil
+
+        expect(tag.to_s).to eq "<tag id=\"my_id\"></tag>\n"
+      end
+
+      context "when there is a nested attribute" do
+        before { tag.build id: "my_id", data: { action: 'some_action' } }
+
+        it "should flatten the attributes when rendering to html" do
+          expect(tag.to_s).to eq "<tag id=\"my_id\" data-action=\"some_action\"></tag>\n"
+        end
+
+        it "shouldn't render attributes that are empty" do
+          tag.class_list # initializes an empty ClassList
+          tag.set_attribute :foo, { bar: '' }
+          tag.set_attribute :bar, { baz: nil }
 
-    it "shouldn't render attributes that are empty" do
-      tag.class_list # initializes an empty ClassList
-      tag.set_attribute :foo, ''
-      tag.set_attribute :bar, nil
+          expect(tag.to_s).to eq "<tag id=\"my_id\" data-action=\"some_action\"></tag>\n"
+        end
+      end
+
+      context "when there is a deeply nested attribute" do
+        before { tag.build id: "my_id", foo: { bar: { baz: 'foozle' } } }
 
-      expect(tag.to_s).to eq "<tag id=\"my_id\"></tag>\n"
+        it "should flatten the attributes when rendering to html" do
+          expect(tag.to_s).to eq "<tag id=\"my_id\" foo-bar-baz=\"foozle\"></tag>\n"
+        end
+      end
+
+      context "when there are multiple nested attributes" do
+        before { tag.build id: "my_id", foo: { bar: 'foozle1', baz: 'foozle2' } }
+
+        it "should flatten the attributes when rendering to html" do
+          expect(tag.to_s).to eq "<tag id=\"my_id\" foo-bar=\"foozle1\" foo-baz=\"foozle2\"></tag>\n"
+        end
+      end
     end
 
     it "should get an attribute value" do