add support for required properties

README.md

@@ -629,6 +629,25 @@ purposes; the exception is generated if `:retired_at` is set to _anything_.)
 You can retire events, categories, and entire versions; this system ensures the DSL continues to be a historical record
 of what things were in the past, as well as what they are today.
 
+### Requiring Properties
+
+Sometimes it is helpful to enforce the presence of certain properties. For example, imagine you are tracking views of
+a set of banners that advertise a set of new features. In order to determine which banners (and therefore features) are
+driving conversions, you need to be sure to include `:banner_id` and `:banner_size`. If there are many places in the
+code that can call this event, you may forget to pass along these properties. With `:required_properties` an exception
+is generated if the keys are not present _or if their values are blank_.
+
+```ruby
+global_events_prefix :ab
+
+version 1, "2014-02-04" do
+  category :user do
+    event :viewed_banner, "2014-02-04", "user creates a brand-new account", :required_properties => [ :banner_id, :banner_size ]
+  end
+end
+```
+
+
 ### Adding Notes to Events
 
 You can also add notes to events. They must be tagged with the author and the time, and they can be very useful for

lib/meta_events/definition/definition_set.rb

@@ -15,6 +15,7 @@ module Definition
     class DefinitionSet
       class BaseError < StandardError; end
       class RetiredEventError < BaseError; end
+      class RequiredPropertyMissingError < BaseError; end
 
       class << self
         # Creates an MetaEvents::Definition::DefinitionSet. +source+ can be one of:

lib/meta_events/definition/event.rb

@@ -73,8 +73,10 @@ def initialize(category, name, *args, &block)
       # provide for required properties, property validation, or anything else.
       def validate!(properties)
         if retired_at
-          raise ::MetaEvents::Definition::DefinitionSet::RetiredEventError, "Event #{full_name} was retired at #{retired_at.inspect} (or its category or version was); you can't use it any longer."
+          raise ::MetaEvents::Definition::DefinitionSet::RetiredEventError,
+            "Event #{full_name} was retired at #{retired_at.inspect} (or its category or version was); you can't use it any longer."
         end
+        validate_properties!(properties)
       end
 
       # Returns, or sets, the description for an event.
@@ -142,17 +144,34 @@ def ensure_complete!
         raise ArgumentError, "You must record when you introduced event #{full_name}, either as an argument, in the options, or using 'introduced'" if (! @introduced)
       end
 
+      def validate_properties!(properties)
+        properties_with_indifferent_access = properties.with_indifferent_access
+        if @required_properties
+          missing_properties = [ ]
+          @required_properties.each do |required_property|
+            if properties_with_indifferent_access[required_property].blank?
+              missing_properties << required_property
+            end
+          end
+          unless missing_properties.empty?
+            raise ::MetaEvents::Definition::DefinitionSet::RequiredPropertyMissingError,
+              "Event #{full_name} requires the properties #{@required_properties.join(', ')}. #{missing_properties.join(', ')} were missing or had blank values."
+          end
+        end
+      end
+
       # Called with the set of options (which can be empty) supplied in the constructor; responsible for applying those
       # to the object properly.
       def apply_options!(options)
-        options.assert_valid_keys(:introduced, :desc, :description, :retired_at, :external_name)
+        options.assert_valid_keys(:introduced, :desc, :description, :retired_at, :external_name, :required_properties)
 
         introduced options[:introduced] if options[:introduced]
         desc options[:desc] if options[:desc]
         desc options[:description] if options[:description]
         external_name options[:external_name] if options[:external_name]
 
         @retired_at = Time.parse(options[:retired_at]) if options[:retired_at]
+        @required_properties = Array(options[:required_properties]) if options[:required_properties]
       end
 
       # Called with the arguments (past the category and event name) supplied to the constructor; responsible for

spec/meta_events/definition/event_spec.rb

@@ -66,6 +66,14 @@
         allow(category).to receive(:retired_at).and_return(Time.parse("2013-02-01"))
         expect { instance.validate!(:foo => :bar) }.to raise_error(::MetaEvents::Definition::DefinitionSet::RetiredEventError, /2013/)
       end
+
+      it "should fail if required properties are missing" do
+        expect { klass.new(category, :foo, "2016-1-1", "foobar", :required_properties => [ :foo ]).validate!(:baz => :bar) }.to raise_error(::MetaEvents::Definition::DefinitionSet::RequiredPropertyMissingError, /foo/)
+      end
+
+      it "should fail if required properties have blank values" do
+        expect { klass.new(category, :foo, "2016-1-1", "foobar", :required_properties => [ :foo ]).validate!(:foo => '') }.to raise_error(::MetaEvents::Definition::DefinitionSet::RequiredPropertyMissingError, /foo/)
+      end
     end
 
     it "should return and allow setting its description via #desc" do
@@ -143,5 +151,15 @@
         expect(instance.external_name).to eq("my name")
       end
     end
+
+    context "with required properties" do
+      it "should work with strings and symbols" do
+        expect do
+          event = klass.new(category, :foo, "2016-1-1", "foobar", :required_properties => [ "string", :symbol ])
+          event.validate!('string' => "foo", :symbol => "foo")
+          event.validate!(:string => "foo", "symbol" => "foo")
+        end.to_not raise_error(::MetaEvents::Definition::DefinitionSet::RequiredPropertyMissingError)
+      end
+    end
   end
 end