Add a custom form helper for displaying collection options

**ISSUE**
We want to be able to provide a collection drop-down selection list
in our collection and item edit forms, but we want it to handle
multivalue fields by displaying individual inputs that aggregate
as an array valued parameter - i.e. each takes a single value and the
field name ends in '[]' - instead of a multi-value select box.

**SOLUTION**
Implement a custom input that provides the desired behaviors for our
forms.

.rubocop.yml

@@ -52,6 +52,10 @@ Style/FrozenStringLiteralComment:
 Style/HashSyntax:
   EnforcedShorthandSyntax: never
 
+Style/OpenStructUse:
+  Exclude:
+    - 'spec/helpers/t3_form_builder_spec.rb'
+
 Style/SymbolProc:
   AllowedPatterns: ['map', 'select']
 

app/helpers/t3_form_builder.rb

@@ -0,0 +1,15 @@
+# Custom form controls to support T3 field-specific data types
+# e.g. #vacabulary provides a customized select populated with a defined vocabulary
+class T3FormBuilder < ActionView::Helpers::FormBuilder
+  def vocabulary(method, options = {})
+    multiple = options.delete(:multiple)
+    selected = options.delete(:value) || ''
+    select_options = options.except(:multiple)
+                            .reverse_merge(
+                              { name: @template.field_name(@object_name, method, multiple: multiple) }
+                            )
+                            .merge({ selected: selected })
+    option_tags = @template.options_from_collection_for_select(Collection.order(:created_at), :id, :id, selected)
+    select(method, option_tags, { prompt: 'Select one', selected: '', disabled: true }, select_options)
+  end
+end

spec/helpers/t3_form_builder_spec.rb

@@ -0,0 +1,50 @@
+require 'rails_helper'
+
+RSpec.describe T3FormBuilder do
+  let(:form_builder) do
+    described_class.new('item[metadata]',
+                        OpenStruct.new({ 'Title' => 'Working Papers' }),
+                        Object.extend(ActionView::Helpers::FormHelper,
+                                      ActionView::Helpers::FormTagHelper, ActionView::Helpers::FormOptionsHelper),
+                        {})
+  end
+  let(:tag_options) { {} }
+
+  describe '#vocabulary' do
+    let(:vocabulary_helper) { Capybara.string(form_builder.vocabulary(:collection, tag_options)) }
+
+    it 'renders a select with the expected id' do
+      expect(vocabulary_helper).to have_select('item[metadata][collection]')
+    end
+
+    it 'displays a prompt when no value is selected' do
+      expect(vocabulary_helper).to have_selector('select option[selected]', text: 'Select one')
+    end
+
+    context 'with options {multipe: true}' do
+      let(:tag_options) { { multiple: true } }
+
+      it 'renders as an array instead of a multi-select' do
+        expect(vocabulary_helper).to have_select('item[metadata][collection][]')
+      end
+    end
+
+    describe 'displays options' do
+      let(:tag_options) { { value: 'Green' } }
+
+      before do
+        collections = [OpenStruct.new({ id: 'Red' }), OpenStruct.new({ id: 'Green' }), OpenStruct.new({ id: 'Blue' })]
+        allow(Collection).to receive(:order).and_return(collections)
+      end
+
+      example 'showing available collections' do
+        select_options = vocabulary_helper.all('option').map(&:text)
+        expect(select_options).to eq ['Select one', 'Red', 'Green', 'Blue']
+      end
+
+      example 'with a selection' do
+        expect(vocabulary_helper).to have_selector('select option[selected]', text: 'Green')
+      end
+    end
+  end
+end