Allow a default value when the attribute is assigned as nil, with the :allow_nil option

README.md

@@ -157,6 +157,10 @@ class Page
 
   # default from a singleton value (boolean in this case)
   attribute :published, Boolean, :default => false
+
+  # default from a singleton value (string in this case),
+  # that doesn't allow a nil value
+  attribute :author, String, :default => 'Mies', :allow_nil => false
 
   # default from a callable object (proc in this case)
   attribute :slug, String, :default => lambda { |page, attribute| page.title.downcase.gsub(' ', '-') }
@@ -169,10 +173,11 @@ class Page
   end
 end
 
-page = Page.new(:title => 'Virtus README')
+page = Page.new(:title => 'Virtus README', :author => nil)
 page.slug         # => 'virtus-readme'
 page.views        # => 0
 page.published    # => false
+page.author       # => 'Mies'
 page.editor_title # => "UNPUBLISHED: Virtus README"
 
 page.views = 10

lib/virtus/attribute.rb

@@ -20,12 +20,13 @@ class Attribute
 
     include ::Equalizer.new(:type, :options)
 
-    accept_options :primitive, :accessor, :default, :lazy, :strict, :required, :finalize
+    accept_options :primitive, :accessor, :default, :lazy, :strict, :required, :finalize, :allow_nil
 
     strict false
     required true
     accessor :public
     finalize true
+    allow_nil true
 
     # @see Virtus.coerce
     #
@@ -210,6 +211,23 @@ def finalized?
       frozen?
     end
 
+    # Return if the attribute accepts nil as value when default value is set
+    #
+    # @example
+    #
+    #   attr = Virtus::Attribute.build(String, :default => 'foo', :allow_nil => true)
+    #   attr.allow_nil? # => true
+    #
+    #   attr = Virtus::Attribute.build(String, :default => 'foo', :allow_nil => false)
+    #   attr.allow_nil? # => false
+    #
+    # @return [Boolean]
+    #
+    # @api public
+    def allow_nil?
+      options[:allow_nil]
+    end
+
     # @api private
     def define_accessor_methods(attribute_set)
       attribute_set.define_reader_method(self, name,       options[:reader])

lib/virtus/attribute/accessor.rb

@@ -34,6 +34,18 @@ def self.extended(descendant)
         descendant.instance_variable_set('@instance_variable_name', "@#{name}")
       end
 
+      # Return if attribute value is something other than
+      # a nil value when attribute does not accept nil values
+      #
+      # @param [Object] instance
+      #
+      # @return [Boolean]
+      #
+      # @api public
+      def present?(instance)
+        self.allow_nil? ? self.defined?(instance) : !instance.instance_variable_get(instance_variable_name).nil?
+      end
+
       # Return if attribute value is defined
       #
       # @param [Object] instance

lib/virtus/attribute/lazy_default.rb

@@ -5,7 +5,7 @@ module LazyDefault
 
       # @api public
       def get(instance)
-        if instance.instance_variable_defined?(instance_variable_name)
+        if present?(instance)
           super
         else
           set_default_value(instance)

lib/virtus/attribute_set.rb

@@ -209,7 +209,7 @@ def finalize
 
     # @api private
     def skip_default?(object, attribute)
-      attribute.lazy? || attribute.defined?(object)
+      attribute.lazy? || attribute.present?(object)
     end
 
     # Merge the attributes into the index

spec/integration/default_values_spec.rb

@@ -15,6 +15,7 @@ class Page
         include Virtus
 
         attribute :title,        String
+        attribute :author,       String,  :default => 'Mies', :allow_nil => false
         attribute :slug,         String,  :default => lambda { |post, attribute| post.title.downcase.gsub(' ', '-') }, :lazy => true
         attribute :view_count,   Integer, :default => 0
         attribute :published,    Boolean, :default => false, :accessor => :private
@@ -59,6 +60,13 @@ def default_editor_title
     end.to change { subject.view_count }.to(0)
   end
 
+  context 'when attribute is assigned as nil' do
+    specify 'you can set a default with the :allow_nil option' do
+      page = Examples::Page.new :author => nil
+      expect(page.author).to eql 'Mies'
+    end
+  end
+
   context 'a ValueObject' do
     it 'does not duplicate the ValueObject' do
       page1 = Examples::Page.new

spec/unit/virtus/attribute/allow_nil_predicate_spec.rb

@@ -0,0 +1,19 @@
+require 'spec_helper'
+
+describe Virtus::Attribute, '#allow_nil?' do
+  subject { object.allow_nil? }
+
+  let(:object) { described_class.build(String, :allow_nil => allow_nil) }
+
+  context 'when allow_nil option is true' do
+    let(:allow_nil) { true }
+
+    it { should be(true) }
+  end
+
+  context 'when allow_nil option is false' do
+    let(:allow_nil) { false }
+
+    it { should be(false) }
+  end
+end

spec/unit/virtus/attribute/present_spec.rb

@@ -0,0 +1,40 @@
+require 'spec_helper'
+
+describe Virtus::Attribute, '#present?' do
+  subject { object.present?(instance) }
+
+  let(:model)    { Class.new { attr_accessor :test } }
+  let(:name)     { :test }
+  let(:instance) { model.new }
+
+  context 'when the attribute allows nil values' do
+    let(:object) { described_class.build(String, :name => name, :allow_nil => true) }
+
+    context 'and the attribute value is defined as nil' do
+      before { instance.test = nil }
+      it { should be(true) }
+    end
+
+    context 'and the attribute value is NOT defined' do
+      it { should be(false) }
+    end
+  end
+
+  context 'when the attribute does NOT allow nil values' do
+    let(:object) { described_class.build(String, :name => name, :allow_nil => false) }
+
+    context 'and the attribute value is defined' do
+      before { instance.test = 'foo' }
+      it { should be(true) }
+    end
+
+    context 'and the attribute value is defined as nil' do
+      before { instance.test = nil }
+      it { should be(false) }
+    end
+
+    context 'and the attribute value is NOT defined' do
+      it { should be(false) }
+    end
+  end
+end

spec/unit/virtus/class_methods/new_spec.rb

@@ -7,7 +7,9 @@
 
       attribute :id,    Integer
       attribute :name,  String, :default => 'John Doe'
+      attribute :alias, String, :default => 'jdoe', :allow_nil => false
       attribute :email, String, :default => '[email protected]', :lazy => true, :writer => :private
+      attribute :age,   Integer, :default => 18, :allow_nil => false, :lazy => true
     }
   }
 
@@ -16,21 +18,31 @@
 
     it 'sets default values for non-lazy attributes' do
       expect(subject.instance_variable_get('@name')).to eql('John Doe')
+      expect(subject.instance_variable_get('@alias')).to eql('jdoe')
     end
 
     it 'skips setting default values for lazy attributes' do
       expect(subject.instance_variable_get('@email')).to be(nil)
+      expect(subject.instance_variable_get('@age')).to be(nil)
     end
   end
 
   context 'with attribute hash' do
-    subject { model.new(:id => 1, :name => 'Jane Doe') }
+    subject { model.new(:id => 1, :name => 'Jane Doe', :alias => nil, :age => nil) }
 
     it 'sets attributes with public writers' do
       expect(subject.id).to be(1)
       expect(subject.name).to eql('Jane Doe')
     end
 
+    it 'sets default values for attributes assigned as nil without allow_nil' do
+      expect(subject.alias).to eql('jdoe')
+    end
+
+    it 'sets default values for lazy attributes assigned as nil without allow_nil' do
+      expect(subject.age).to eql(18)
+    end
+
     it 'skips setting attributes with private writers' do
       expect(subject.instance_variable_get('@email')).to be(nil)
     end