Fixed "Hash#to_param confused by empty hash values #13892"

activesupport/lib/active_support/core_ext/object/to_param.rb

@@ -51,6 +51,7 @@ class Hash
   #
   # This method is also aliased as +to_query+.
   def to_param(namespace = nil)
+    return (namespace ? nil.to_query(namespace) : '') if empty?
     collect do |key, value|
       value.to_query(namespace ? "#{namespace}[#{key}]" : key)
     end.sort! * '&'

activesupport/test/core_ext/object/to_param_test.rb

 require 'abstract_unit'
 require 'active_support/core_ext/object/to_param'
+require 'active_support/core_ext/object/to_query'
 
 class ToParamTest < ActiveSupport::TestCase
   def test_object
@@ -16,4 +17,14 @@ def test_boolean
     assert_equal true, true.to_param
     assert_equal false, false.to_param
   end
+
+  def test_nested_empty_hash
+    hash1 = {a: 1, b: {c: 3, d: {}}}.to_param
+    hash2 = {p: 12, b: {c: 3, e: nil, f: ''}}.to_param
+    hash3 = {b: {c: 3, k: {}, f: '' }}.to_param
+
+    assert_equal 'a=1&b[c]=3&b[d]=', CGI::unescape(hash1)
+    assert_equal 'b[c]=3&b[e]=&b[f]=&p=12', CGI::unescape(hash2)
+    assert_equal 'b[c]=3&b[f]=&b[k]=', CGI::unescape(hash3)
+  end
 end