Move field requireness logic to Field::Base

Given that application helpers are available throughout the app and the
highly specific nature of some of the actions required to determine
whether or not a field is required, moving that logic to Field::Base
feels more appropriate.

This also allows us to add more complex checks to determine whether or
not a field is required without exposing that logic everywhere in the
application.

app/helpers/administrate/application_helper.rb

@@ -17,22 +17,7 @@ def render_field(field, locals = {})
     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)
+      field.required? ? "required" : "optional"
     end
 
     def dashboard_from_resource(resource_name)

lib/administrate/field/base.rb

@@ -48,6 +48,14 @@ def to_partial_path
         "/fields/#{self.class.field_type}/#{page}"
       end
 
+      def required?
+        return false unless resource.class.respond_to?(:validators_on)
+
+        resource.class.validators_on(attribute).any? do |v|
+          v.class == ActiveRecord::Validations::PresenceValidator
+        end
+      end
+
       attr_reader :attribute, :data, :options, :page, :resource
     end
   end

spec/helpers/administrate/application_helper_spec.rb

@@ -94,18 +94,6 @@
     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")