Add 'default_thumbnail' view configuration

The default thumbnail can be:

- an image asset
- the name of a helper method (as a symbol) that returns the markup for a thumbnail
- a lambda, which will receive the document and returns the thumbnail markup

app/presenters/blacklight/thumbnail_presenter.rb

@@ -18,7 +18,9 @@ def initialize(document, view_context, view_config)
     #
     # @return [Boolean]
     def exists?
-      thumbnail_method.present? || thumbnail_field && document.has?(thumbnail_field)
+      thumbnail_method.present? ||
+        (thumbnail_field && thumbnail_value_from_document(document).present?) ||
+        default_thumbnail.present?
     end
 
     ##
@@ -36,16 +38,35 @@ def thumbnail_tag image_options = {}, url_options = {}
 
     private
 
-    delegate :thumbnail_field, :thumbnail_method, to: :view_config
+    delegate :thumbnail_field, :thumbnail_method, :default_thumbnail, to: :view_config
 
     # @param [Hash] image_options to pass to the image tag
     def thumbnail_value(image_options)
-      if thumbnail_method
-        view_context.send(thumbnail_method, document, image_options)
-      elsif thumbnail_field
-        image_url = Array(thumbnail_field).lazy.map { |field| document.first(field) }.reject(&:blank?).first
-        view_context.image_tag image_url, image_options if image_url.present?
+      value = if thumbnail_method
+                view_context.send(thumbnail_method, document, image_options)
+              elsif thumbnail_field
+                image_url = thumbnail_value_from_document(document)
+                view_context.image_tag image_url, image_options if image_url.present?
+              end
+
+      value || default_thumbnail_value(image_options)
+    end
+
+    def default_thumbnail_value(image_options)
+      return unless default_thumbnail
+
+      case default_thumbnail
+      when Symbol
+        view_context.send(default_thumbnail, document, image_options)
+      when Proc
+        default_thumbnail.call(document, image_options)
+      else
+        view_context.image_tag default_thumbnail, image_options
       end
     end
+
+    def thumbnail_value_from_document(document)
+      Array(thumbnail_field).lazy.map { |field| document.first(field) }.reject(&:blank?).first
+    end
   end
 end

spec/helpers/catalog_helper_spec.rb

@@ -200,17 +200,20 @@ def render_grouped_response?
       expect(Deprecation).to receive(:warn)
     end
 
-    let(:document) { instance_double(SolrDocument) }
+    let(:document) { SolrDocument.new(data) }
+    let(:data) { {} }
 
     it "has a thumbnail if a thumbnail_method is configured" do
       allow(helper).to receive_messages(:blacklight_config => Blacklight::Configuration.new(:index => Blacklight::OpenStructWithHashAccess.new(:thumbnail_method => :xyz, document_presenter_class: Blacklight::IndexPresenter) ))
       expect(helper.has_thumbnail? document).to be true
     end
 
-    it "has a thumbnail if a thumbnail_field is configured and it exists in the document" do
-      allow(helper).to receive_messages(:blacklight_config => Blacklight::Configuration.new(:index => Blacklight::OpenStructWithHashAccess.new(:thumbnail_field => :xyz, document_presenter_class: Blacklight::IndexPresenter) ))
-      allow(document).to receive_messages(has?: true)
-      expect(helper.has_thumbnail? document).to be true
+    context "if a thumbnail_field is configured and it exists in the document" do
+      let(:data) { { xyz: 'abc' } }
+      it "has a thumbnail" do
+        allow(helper).to receive_messages(:blacklight_config => Blacklight::Configuration.new(:index => Blacklight::OpenStructWithHashAccess.new(:thumbnail_field => :xyz, document_presenter_class: Blacklight::IndexPresenter) ))
+        expect(helper.has_thumbnail? document).to be true
+      end
     end
 
     it "does not have a thumbnail if the thumbnail_field is missing from the document" do

spec/presenters/thumbnail_presenter_spec.rb

@@ -45,6 +45,16 @@
       end
     end
 
+    context "when default_thumbnail is configured" do
+      let(:config) do
+        Blacklight::OpenStructWithHashAccess.new(default_thumbnail: 'image.png')
+      end
+
+      context "and the field exists in the document" do
+        it { is_expected.to be true }
+      end
+    end
+
     context "without any configured options" do
       it { is_expected.to be_falsey }
     end
@@ -120,6 +130,40 @@
       end
     end
 
+    context "when default_thumbnail is configured" do
+      context "and is a string" do
+        let(:config) do
+          Blacklight::OpenStructWithHashAccess.new(default_thumbnail: 'image.png')
+        end
+
+        it "creates an image tag for the given asset" do
+          allow(view_context).to receive(:image_tag).with('image.png', {}).and_return('<img src="image.jpg">')
+          expect(presenter.thumbnail_tag({}, suppress_link: true)).to eq '<img src="image.jpg">'
+        end
+      end
+
+      context "and is a symbol" do
+        let(:config) do
+          Blacklight::OpenStructWithHashAccess.new(default_thumbnail: :get_a_default_thumbnail)
+        end
+
+        it "calls that helper method" do
+          allow(view_context).to receive(:get_a_default_thumbnail).with(document, {}).and_return('<img src="image.jpg">')
+          expect(presenter.thumbnail_tag({}, suppress_link: true)).to eq '<img src="image.jpg">'
+        end
+      end
+
+      context "and is a proc" do
+        let(:config) do
+          Blacklight::OpenStructWithHashAccess.new(default_thumbnail: lambda { |_, _| '<img src="image.jpg">' })
+        end
+
+        it "calls that lambda" do
+          expect(presenter.thumbnail_tag({}, suppress_link: true)).to eq '<img src="image.jpg">'
+        end
+      end
+    end
+
     context "when no thumbnail is configured" do
       it { is_expected.to be_nil }
     end