Support for virtual fields

lib/administrate/field/base.rb

@@ -38,6 +38,7 @@ def initialize(attribute, data, page, options = {})
         @page = page
         @resource = options.delete(:resource)
         @options = options
+        @data = read_value if @data.nil?
       end
 
       def html_class
@@ -52,6 +53,18 @@ def name
         attribute.to_s
       end
 
+      def read_value
+        if options.key?(:getter)
+          if options[:getter].respond_to?(:call)
+            options[:getter].call(self)
+          else
+            resource&.public_send(options[:getter])
+          end
+        else
+          resource&.public_send(attribute)
+        end
+      end
+
       def to_partial_path
         "/fields/#{self.class.field_type}/#{page}"
       end

lib/administrate/field/deferred.rb

@@ -21,6 +21,10 @@ def ==(other)
           options == other.options
       end
 
+      def getter
+        options.fetch(:getter, nil)
+      end
+
       def associative?
         deferred_class.associative?
       end
@@ -57,6 +61,10 @@ def permitted_attribute(attr, opts = {})
         end
       end
 
+      def read_value(resource, attribute_name)
+        @deferred_class.read_value(resource, attribute_name, options)
+      end
+
       delegate :html_class, to: :deferred_class
     end
   end

lib/administrate/page/base.rb

@@ -30,13 +30,8 @@ def item_associations
       private
 
       def attribute_field(dashboard, resource, attribute_name, page)
-        value = get_attribute_value(resource, attribute_name)
         field = dashboard.attribute_type_for(attribute_name)
-        field.new(attribute_name, value, page, resource: resource)
-      end
-
-      def get_attribute_value(resource, attribute_name)
-        resource.public_send(attribute_name)
+        field.new(attribute_name, nil, page, resource: resource)
       end
 
       attr_reader :dashboard, :options

spec/example_app/app/dashboards/customer_dashboard.rb

@@ -8,6 +8,10 @@ class CustomerDashboard < Administrate::BaseDashboard
     email_subscriber: Field::Boolean,
     lifetime_value: Field::Number.with_options(prefix: "$", decimals: 2),
     name: Field::String,
+    nickname: Field::String.with_options(
+      getter: ->(field) { "Mr. #{field.resource.name}man" if field.resource.name.present? },
+      searchable: false
+    ),
     orders: Field::HasMany.with_options(limit: 2, sort_by: :id),
     log_entries: Field::HasManyVariant.with_options(limit: 2, sort_by: :id),
     updated_at: Field::DateTime,

spec/lib/fields/base_spec.rb

@@ -167,4 +167,34 @@
       expect(field.required?).to eq(false)
     end
   end
+
+  describe "read_value" do
+    it "reads the value from the resource" do
+      resource = double(attribute: "value")
+      field = field_class.new(:attribute, :date, :page, resource: resource)
+
+      expect(field.read_value).to eq("value")
+    end
+
+    it "reads the value from the resource with a custom getter" do
+      resource = double(custom_getter: "value")
+      field = field_class.new(:attribute, :date, :page, resource: resource, getter: :custom_getter)
+
+      expect(field.read_value).to eq("value")
+    end
+
+    it "reads the value from the resource with a custom getter block" do
+      resource = double
+      field = field_class.new(:attribute, :date, :page, resource: resource, getter: ->(field) { field.resource.custom_getter })
+
+      expect(resource).to receive(:custom_getter)
+      field.read_value
+    end
+
+    it "returns nil if the resource is nil" do
+      field = field_class.new(:attribute, :date, :page, resource: nil)
+
+      expect(field.read_value).to eq(nil)
+    end
+  end
 end