MultiValueInput should pass an index to build_text_field

app/inputs/multi_value_input.rb

@@ -9,10 +9,10 @@ def input(wrapper_options)
         <ul class="listing">
     HTML
 
-    collection.each do |value|
+    collection.each_with_index do |value, index|
       markup << <<-HTML
         <li class="field-wrapper">
-          #{build_text_field(value)}
+          #{build_text_field(value, index)}
         </li>
       HTML
     end
@@ -25,7 +25,10 @@ def input(wrapper_options)
 
   private
 
-    def build_text_field(value)
+    # Although the 'index' parameter is not used in this implementation it is useful in an
+    # an overridden version of this method, especially when the field is a complex object and
+    # the override defines nested fields.
+    def build_text_field(value, index)
       options = input_html_options.dup
 
       options[:value] = value

spec/inputs/multi_value_input_spec.rb

@@ -43,4 +43,19 @@ def [](val)
       expect(subject).to have_selector('.form-group.foo_bar.multi_value ul.listing li input.foo_bar')
     end
   end
+
+  describe '#build_text_field' do
+    let(:foo) { Foo.new }
+    before { foo.bar = ['bar1', 'bar2'] }
+    let(:builder) { double("builder", object: foo, object_name: 'foo') }
+
+    subject { MultiValueInput.new(builder, :bar, nil, :multi_value, {}) }
+
+    it 'renders multi-value' do
+      expect(subject).to receive(:build_text_field).with('bar1', 0)
+      expect(subject).to receive(:build_text_field).with('bar2', 1)
+      expect(subject).to receive(:build_text_field).with('', 2)
+      subject.input({})
+    end
+  end
 end