Raise a helpful error for missing dashboard attrs

Closes #246

Problem:

If an attribute is missing from a dashboard's `ATTRIBUTE_TYPES` hash,
the user will see a generic error
when they visit a dashboard page that tries to display that attribute.

The error is:

```
NoMethodError: undefined method new for `nil:NilClass` in `Page::Base`
```

Solution:

Add accessor methods to `BaseDashboard`
for fetching attributes from the dashboard.
If a requested attribute is not present,
the accessor methods raise a helpful error message.

CHANGELOG.md

@@ -17,6 +17,8 @@
 
 ### Upcoming Release
 
+* [#251] [FEATURE] Raise a helpful error when an attribute is missing from
+  `ATTRIBUTE_TYPES`
 * [#247] [BUGFIX] Populate `has_many` and `belongs_to` select boxes
   with the current value of the relationship.
 * [#237] [I18n] Fix broken paths for several I18n files (de, es, fr, pt-BR, vi).

lib/administrate/base_dashboard.rb

@@ -18,6 +18,18 @@ def attribute_types
       self.class::ATTRIBUTE_TYPES
     end
 
+    def attribute_type_for(attribute_name)
+      attribute_types.fetch(attribute_name) do
+        fail attribute_not_found_message(attribute_name)
+      end
+    end
+
+    def attribute_types_for(attribute_names)
+      attribute_names.each_with_object({}) do |name, attributes|
+        attributes[name] = attribute_type_for(name)
+      end
+    end
+
     def form_attributes
       self.class::FORM_ATTRIBUTES
     end
@@ -39,5 +51,11 @@ def collection_attributes
     def display_resource(resource)
       "#{resource.class} ##{resource.id}"
     end
+
+    private
+
+    def attribute_not_found_message(attr)
+      "Attribute #{attr} could not be found in #{self.class}::ATTRIBUTE_TYPES"
+    end
   end
 end

lib/administrate/page/base.rb

@@ -15,10 +15,8 @@ def resource_name
 
       def attribute_field(dashboard, resource, attribute_name, page)
         value = get_attribute_value(resource, attribute_name)
-
-        dashboard.
-          attribute_types[attribute_name].
-          new(attribute_name, value, page)
+        field = dashboard.attribute_type_for(attribute_name)
+        field.new(attribute_name, value, page)
       end
 
       def get_attribute_value(resource, attribute_name)

lib/administrate/page/collection.rb

@@ -14,7 +14,7 @@ def attributes_for(resource)
       end
 
       def attribute_types
-        dashboard.attribute_types.slice(*attribute_names)
+        dashboard.attribute_types_for(attribute_names)
       end
 
       def ordered_html_class(attr)

spec/dashboards/customer_dashboard_spec.rb

@@ -24,6 +24,49 @@
     end
   end
 
+  def missing_attribute_message(attribute, dashboard_class)
+    "Attribute #{attribute} could not be found in #{dashboard_class}::ATTRIBUTE_TYPES"
+  end
+
+  describe "#attribute_type_for" do
+    context "for an existing attribute" do
+      it "returns the attribute field" do
+        dashboard = CustomerDashboard.new
+        field = dashboard.attribute_type_for(:name)
+        expect(field).to eq Administrate::Field::String
+      end
+    end
+
+    context "for a non-existent attribute" do
+      it "raises an exception" do
+        dashboard = CustomerDashboard.new
+        expect { dashboard.attribute_type_for(:foo) }.
+          to raise_error missing_attribute_message("foo", "CustomerDashboard")
+      end
+    end
+  end
+
+  describe "#attribute_types_for" do
+    context "for existing attributes" do
+      it "returns the attribute fields" do
+        dashboard = CustomerDashboard.new
+        fields = dashboard.attribute_types_for([:name, :email])
+        expect(fields).to match(
+          name: Administrate::Field::String,
+          email: Administrate::Field::Email,
+        )
+      end
+    end
+
+    context "for one non-existent attribute" do
+      it "raises an exception" do
+        dashboard = CustomerDashboard.new
+        expect { dashboard.attribute_types_for([:name, :foo]) }.
+          to raise_error missing_attribute_message("foo", "CustomerDashboard")
+      end
+    end
+  end
+
   describe "#display_resource" do
     it "returns the customer's name" do
       customer = double(name: "Example Customer")