Add the qualifier allow_blank to validate_length_of

lib/shoulda/matchers/active_model/validate_inclusion_of_matcher.rb

@@ -302,11 +302,6 @@ def in_range(range)
           self
         end
 
-        def allow_blank(allow_blank = true)
-          @options[:allow_blank] = allow_blank
-          self
-        end
-
         def allow_nil(allow_nil = true)
           @options[:allow_nil] = allow_nil
           self
@@ -362,20 +357,11 @@ def matches_for_range?
 
         def matches_for_array?
           allows_all_values_in_array? &&
-            allows_blank_value? &&
+            allow_blank_matches? &&
             allows_nil_value? &&
             disallows_value_outside_of_array?
         end
 
-        def allows_blank_value?
-          if @options.key?(:allow_blank)
-            blank_values = ['', ' ', "\n", "\r", "\t", "\f"]
-            @options[:allow_blank] == blank_values.all? { |value| allows_value_of(value) }
-          else
-            true
-          end
-        end
-
         def allows_nil_value?
           if @options.key?(:allow_nil)
             @options[:allow_nil] == allows_value_of(nil)

lib/shoulda/matchers/active_model/validate_length_of_matcher.rb

@@ -216,6 +216,27 @@ module ActiveModel
       #         with_long_message('Secret key must be less than 100 characters')
       #     end
       #
+      # ##### allow_blank
+      #
+      # Use `allow_blank` to assert that the attribute allows blank.
+      #
+      #     class User
+      #       include ActiveModel::Model
+      #       attr_accessor :bio
+      #
+      #       validates_length_of :bio, minimum: 15, allow_blank: true
+      #     end
+      #
+      #     # RSpec
+      #     describe User do
+      #       it { should validate_length_of(:bio).is_at_least(15).allow_blank }
+      #     end
+      #
+      #     # Test::Unit
+      #     class UserTest < ActiveSupport::TestCase
+      #       should validate_length_of(:bio).is_at_least(15).allow_blank
+      #     end
+      #
       # @return [ValidateLengthOfMatcher]
       #
       def validate_length_of(attr)
@@ -293,7 +314,7 @@ def description
         def matches?(subject)
           super(subject)
           translate_messages!
-          lower_bound_matches? && upper_bound_matches?
+          lower_bound_matches? && upper_bound_matches? && allow_blank_matches?
         end
 
         private

lib/shoulda/matchers/active_model/validation_matcher.rb

@@ -19,6 +19,11 @@ def on(context)
           self
         end
 
+        def allow_blank
+          @options[:allow_blank] = true
+          self
+        end
+
         def strict
           @strict = true
           self
@@ -95,6 +100,15 @@ def disallow_value_matcher(value, message)
         def strict?
           @strict
         end
+
+        def allow_blank_matches?
+          if @options.key?(:allow_blank)
+            blank_values = ['', ' ', "\n", "\r", "\t", "\f"]
+            @options[:allow_blank] == blank_values.all? { |value| allows_value_of(value) }
+          else
+            true
+          end
+        end
       end
     end
   end

spec/unit/shoulda/matchers/active_model/validate_length_of_matcher_spec.rb

@@ -161,6 +161,27 @@
     end
   end
 
+  context 'qualified with allow_blank' do
+    context 'and validating with allow_blank' do
+      it 'accepts' do
+        expect(validating_length(minimum: 2, allow_blank: true)).
+          to validate_length_of(:attr).is_at_least(2).allow_blank
+      end
+    end
+
+    context 'and not validating with allow_blank' do
+      it 'rejects' do
+        assertion = lambda do
+          expect(validating_length(minimum: 2)).
+            to validate_length_of(:attr).is_at_least(2).allow_blank
+        end
+        expect(&assertion).to fail_with_message_including(
+          %[Did not expect errors when attr is set to ""]
+        )
+      end
+    end
+  end
+
   def validating_length(options = {})
     define_model(:example, attr: :string) do
       validates_length_of :attr, options