Support for maxlength on string inputs inferred from validation

lib/simple_form/inputs/string_input.rb

@@ -8,11 +8,12 @@ class StringInput < Base
 
       def input
         input_html_options[:size]      ||= [limit, SimpleForm.default_input_size].compact.min
+        preferred_max_length = maximum_length_from_validation
+        input_html_options[:maxlength] ||= preferred_max_length if preferred_max_length
         input_html_options[:maxlength] ||= limit if limit && SimpleForm.html5
         if password? || SimpleForm.html5
           input_html_options[:type]    ||= input_type unless string?
         end
-
         @builder.send(input_method, attribute_name, input_html_options)
       end
 
@@ -37,6 +38,17 @@ def string?
       def password?
         input_type == :password
       end
+
+      def maximum_length_from_validation
+        return unless has_validators?
+
+        length_validator = find_length_validator or return
+        length_validator.options[:maximum] if length_validator.options.key? :maximum
+      end
+
+      def find_length_validator
+        attribute_validators.find { |v| ActiveModel::Validations::LengthValidator === v }
+      end
     end
   end
 end

test/inputs_test.rb

@@ -166,6 +166,11 @@ def with_input_for(object, attribute_name, type, options={})
     assert_select 'input.password[type=password][maxlength=100]'
   end
 
+  test 'input should infer maxlength column definition from validation when present' do
+    with_input_for @validating_user, :name, :string
+    assert_select 'input.string[maxlength=25]'
+  end
+
   test 'when not using HTML5, does not show maxlength attribute' do
     swap SimpleForm, :html5 => false do
       with_input_for @user, :password, :password

test/support/models.rb

@@ -141,6 +141,7 @@ class ValidatingUser < User
     :greater_than_or_equal_to => :min_attempts,
     :less_than_or_equal_to => :max_attempts,
     :only_integer => true
+  validates_length_of :name, :maximum => 25
 
   def min_amount
     10