3 files changed, 43 insertions, 2 deletions
diff --git a/activemodel/CHANGELOG.md b/activemodel/CHANGELOG.md
index 048c43f2c4..82e3a7f4dd 100644
--- a/activemodel/CHANGELOG.md
+++ b/activemodel/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Allow passing a Proc or Symbol to length validator options.
+
+    *Matt Rohrer*
+
 *   Add method `#merge!` for `ActiveModel::Errors`.
 
     *Jahfer Husain*
diff --git a/activemodel/lib/active_model/validations/length.rb b/activemodel/lib/active_model/validations/length.rb
index d1a4197286..d6c80b2c5d 100644
--- a/activemodel/lib/active_model/validations/length.rb
+++ b/activemodel/lib/active_model/validations/length.rb
@@ -31,8 +31,8 @@ module ActiveModel
         keys.each do |key|
           value = options[key]
 
-          unless (value.is_a?(Integer) && value >= 0) || value == Float::INFINITY
-            raise ArgumentError, ":#{key} must be a nonnegative Integer or Infinity"
+          unless (value.is_a?(Integer) && value >= 0) || value == Float::INFINITY || value.is_a?(Symbol) || value.is_a?(Proc)
+            raise ArgumentError, ":#{key} must be a nonnegative Integer, Infinity, Symbol, or Proc"
           end
         end
       end
@@ -45,6 +45,12 @@ module ActiveModel
           next unless check_value = options[key]
 
           if !value.nil? || skip_nil_check?(key)
+            case check_value
+            when Proc
+              check_value = check_value.call(record)
+            when Symbol
+              check_value = record.send(check_value)
+            end
             next if value_length.send(validity_check, check_value)
           end
 
diff --git a/activemodel/test/cases/validations/length_validation_test.rb b/activemodel/test/cases/validations/length_validation_test.rb
index a0d8e058f5..42f76f3e3c 100644
--- a/activemodel/test/cases/validations/length_validation_test.rb
+++ b/activemodel/test/cases/validations/length_validation_test.rb
@@ -410,4 +410,35 @@ class LengthValidationTest < ActiveModel::TestCase
     assert Topic.new("title" => "david").valid?
     assert Topic.new("title" => "david2").invalid?
   end
+
+  def test_validates_length_of_using_proc_as_maximum
+    Topic.validates_length_of :title, maximum: ->(model) { 5 }
+
+    t = Topic.new("title" => "valid", "content" => "whatever")
+    assert t.valid?
+
+    t.title = "notvalid"
+    assert t.invalid?
+    assert t.errors[:title].any?
+    assert_equal ["is too long (maximum is 5 characters)"], t.errors[:title]
+
+    t.title = ""
+    assert t.valid?
+  end
+
+  def test_validates_length_of_using_proc_as_maximum_with_model_method
+    Topic.send(:define_method, :max_title_length, lambda { 5 })
+    Topic.validates_length_of :title, maximum: Proc.new(&:max_title_length)
+
+    t = Topic.new("title" => "valid", "content" => "whatever")
+    assert t.valid?
+
+    t.title = "notvalid"
+    assert t.invalid?
+    assert t.errors[:title].any?
+    assert_equal ["is too long (maximum is 5 characters)"], t.errors[:title]
+
+    t.title = ""
+    assert t.valid?
+  end
 end