Expose required fields on form (#1521)

Displays an asterisk beside label on form when field is required

Issue: #1489

![Captura de Tela 2020-01-09 às 17 20 38](https://user-images.githubusercontent.com/54637230/72172195-55363580-33b3-11ea-8475-edb4520b276d.png)

app/assets/stylesheets/administrate/components/_field-unit.scss

@@ -34,3 +34,10 @@
     width: calc(25% - 1rem);
   }
 }
+
+.field-unit--required {
+  label::after {
+    color: $red;
+    content: " *";
+  }
+}

app/helpers/administrate/application_helper.rb

@@ -15,6 +15,25 @@ def render_field(field, locals = {})
       render locals: locals, partial: field.to_partial_path
     end
 
+    def requireness(field)
+      required_field?(field) ? "required" : "optional"
+    end
+
+    def required_field?(field)
+      has_presence_validator?(field.resource.class, field.attribute)
+    end
+
+    def has_presence_validator?(resource_class, field_name)
+      validators_on(resource_class, field_name).
+        any? { |v| v.class == ActiveRecord::Validations::PresenceValidator }
+    end
+
+    def validators_on(resource_class, field_name)
+      return [] unless resource_class.respond_to?(:validators_on)
+
+      resource_class.validators_on(field_name)
+    end
+
     def class_from_resource(resource_name)
       resource_name.to_s.classify.constantize
     end

app/views/administrate/application/_form.html.erb

@@ -34,7 +34,7 @@ and renders all form fields for a resource's editable attributes.
   <% end %>
 
   <% page.attributes.each do |attribute| -%>
-    <div class="field-unit field-unit--<%= attribute.html_class %>">
+    <div class="field-unit field-unit--<%= attribute.html_class %> field-unit--<%= requireness(attribute) %>">
       <%= render_field attribute, f: f %>
     </div>
   <% end -%>

spec/helpers/administrate/application_helper_spec.rb

@@ -54,6 +54,34 @@
     end
   end
 
+  describe "#requireness" do
+    let(:page) do
+      Administrate::Page::Form.new(Blog::PostDashboard.new, Blog::Post.new)
+    end
+
+    it "returns 'required' if field is required" do
+      title = page.attributes.detect { |i| i.attribute == :title }
+      expect(requireness(title)).to eq("required")
+    end
+
+    it "returns 'optional' if field is not required" do
+      publish_at = page.attributes.detect { |i| i.attribute == :published_at }
+      expect(requireness(publish_at)).to eq("optional")
+    end
+  end
+
+  describe "#has_presence_validator?" do
+    it "returns true if field is required" do
+      required = has_presence_validator?(Blog::Post, :title)
+      expect(required).to eq(true)
+    end
+
+    it "returns false if field is not required" do
+      required = has_presence_validator?(Blog::Post, :publish_at)
+      expect(required).to eq(false)
+    end
+  end
+
   describe "#sort_order" do
     it "sanitizes to ascending/descending/none" do
       expect(sort_order("asc")).to eq("ascending")