travel/travel_to travel time helpers, now raise on nested calls,

     as this can lead to confusing time stubbing.

     Instead of:

         travel_to 2.days.from_now do
           # 2 days from today
           travel_to 3.days.from_now do
             # 5 days from today
           end
         end

     preferred way to achieve above is:

         travel_to 2.days.from_now
         # 2 days from today

         travel_back
         travel_to 5.days.from_now
         # 5 days from today

Closes rails#24690
Fixes rails#24689

activerecord/test/cases/mixin_test.rb

@@ -41,13 +41,12 @@ def test_many_updates
 
     old_updated_at = stamped.updated_at
 
-    travel 5.minutes do
-      stamped.lft_will_change!
-      stamped.save
+    travel 5.minutes
+    stamped.lft_will_change!
+    stamped.save
 
-      assert_equal Time.now, stamped.updated_at
-      assert_equal old_updated_at, stamped.created_at
-    end
+    assert_equal Time.now, stamped.updated_at
+    assert_equal old_updated_at, stamped.created_at
   end
 
   def test_create_turned_off

activesupport/CHANGELOG.md

@@ -1,3 +1,28 @@
+*   `travel/travel_to` travel time helpers, now raise on nested calls, 
+     as this can lead to confusing time stubbing.
+
+     Instead of:
+
+         travel_to 2.days.from_now do
+           # 2 days from today
+           travel_to 3.days.from_now do
+             # 5 days from today
+           end          
+         end
+
+     preferred way to achieve above is:
+
+         travel 2.days do 
+           # 2 days from today
+         end
+         
+         travel 5.days do  
+           # 5 days from today          
+         end        
+        
+     *Vipul A M*
+
+
 *   Support parsing JSON time in ISO8601 local time strings in
     `ActiveSupport::JSON.decode` when `parse_json_times` is enabled.
     Strings in the format of `YYYY-MM-DD hh:mm:ss` (without a `Z` at

activesupport/lib/active_support/testing/time_helpers.rb

@@ -1,32 +1,39 @@
+require 'active_support/core_ext/string/strip' # for strip_heredoc
+require 'concurrent/map'
+
 module ActiveSupport
   module Testing
     class SimpleStubs # :nodoc:
       Stub = Struct.new(:object, :method_name, :original_method)
 
       def initialize
-        @stubs = {}
+        @stubs = Concurrent::Map.new { |h, k| h[k] = {} }
       end
 
       def stub_object(object, method_name, return_value)
-        key = [object.object_id, method_name]
-
-        if stub = @stubs[key]
+        if stub = stubbing(object, method_name)
           unstub_object(stub)
         end
 
         new_name = "__simple_stub__#{method_name}"
 
-        @stubs[key] = Stub.new(object, method_name, new_name)
+        @stubs[object.object_id][method_name] = Stub.new(object, method_name, new_name)
 
         object.singleton_class.send :alias_method, new_name, method_name
         object.define_singleton_method(method_name) { return_value }
       end
 
       def unstub_all!
-        @stubs.each_value do |stub|
-          unstub_object(stub)
+        @stubs.each_value do |object_stubs|
+          object_stubs.each_value do |stub|
+            unstub_object(stub)
+          end
         end
-        @stubs = {}
+        @stubs.clear
+      end
+
+      def stubbing(object, method_name)
+        @stubs[object.object_id][method_name]
       end
 
       private
@@ -93,6 +100,34 @@ def travel(duration, &block)
       #   end
       #   Time.current # => Sat, 09 Nov 2013 15:34:49 EST -05:00
       def travel_to(date_or_time)
+        if block_given? && simple_stubs.stubbing(Time, :now)
+          travel_to_nested_block_call = <<-MSG.strip_heredoc
+
+      Calling `travel_to` with a block, when we have previously already made a call to `travel_to`, can lead to confusing time stubbing.
+
+      Instead of:
+
+         travel_to 2.days.from_now do
+           # 2 days from today
+           travel_to 3.days.from_now do
+             # 5 days from today
+           end
+         end
+
+      preferred way to achieve above is:
+
+         travel 2.days do
+           # 2 days from today
+         end
+
+         travel 5.days do
+           # 5 days from today
+         end
+
+          MSG
+          raise travel_to_nested_block_call
+        end
+
         if date_or_time.is_a?(Date) && !date_or_time.is_a?(DateTime)
           now = date_or_time.midnight.to_time
         else

activesupport/test/time_travel_test.rb

@@ -77,6 +77,49 @@ def test_time_helper_travel_back
     end
   end
 
+  def test_time_helper_travel_to_with_nested_calls_with_blocks
+    Time.stub(:now, Time.now) do
+      outer_expected_time = Time.new(2004, 11, 24, 01, 04, 44)
+      inner_expected_time = Time.new(2004, 10, 24, 01, 04, 44)
+      travel_to outer_expected_time do
+        assert_raises(RuntimeError, /Calling `travel_to` with a block, when we have previously already made a call to `travel_to`, can lead to confusing time stubbing./) do
+          travel_to(inner_expected_time) do
+            #noop
+          end
+        end
+      end
+    end
+  end
+
+  def test_time_helper_travel_to_with_nested_calls
+    Time.stub(:now, Time.now) do
+      outer_expected_time = Time.new(2004, 11, 24, 01, 04, 44)
+      inner_expected_time = Time.new(2004, 10, 24, 01, 04, 44)
+      travel_to outer_expected_time do
+        assert_nothing_raised do
+          travel_to(inner_expected_time)
+
+          assert_equal inner_expected_time, Time.now
+        end
+      end
+    end
+  end
+
+  def test_time_helper_travel_to_with_subsequent_calls
+    Time.stub(:now, Time.now) do
+      initial_expected_time = Time.new(2004, 11, 24, 01, 04, 44)
+      subsequent_expected_time = Time.new(2004, 10, 24, 01, 04, 44)
+      assert_nothing_raised do
+        travel_to initial_expected_time
+        travel_to subsequent_expected_time
+
+        assert_equal subsequent_expected_time, Time.now
+
+        travel_back
+      end
+    end
+  end
+
   def test_travel_to_will_reset_the_usec_to_avoid_mysql_rouding
     Time.stub(:now, Time.now) do
       travel_to Time.utc(2014, 10, 10, 10, 10, 50, 999999) do