Keep the :fallback null object out of counter cache callbacks

The counter cache after_create/after_update/before_destroy callbacks read
the parent with __send__(name) to adjust its counter. With a :fallback
that returned the null object when no parent was set, and the callbacks
then indexed into it (record[cache_column]), raising NoMethodError on
create, update, or destroy.

Read through without_autobuild so the callbacks operate on the real
parent and no-op when there is none, matching how the cascade helpers in
Depending read associations.

Author: cperezabo

lib/mongoid/association/referenced/counter_cache.rb

@@ -110,7 +110,7 @@ def self.define_callbacks!(association)
                   end
                 end
 
-                if (record = __send__(name)) && !current.nil?
+                if (record = without_autobuild { __send__(name) }) && !current.nil?
                   record[cache_column] = (record[cache_column] || 0) + 1
                   record.class.with(record.persistence_context) do |_class|
                     _class.increment_counter(cache_column, current) if record.persisted?
@@ -120,7 +120,7 @@ def self.define_callbacks!(association)
             end
 
             klass.after_create do
-              if record = __send__(name)
+              if record = without_autobuild { __send__(name) }
                 record[cache_column] = (record[cache_column] || 0) + 1
 
                 if record.persisted?
@@ -133,7 +133,7 @@ def self.define_callbacks!(association)
             end
 
             klass.before_destroy do
-              if record = __send__(name)
+              if record = without_autobuild { __send__(name) }
                 record[cache_column] = (record[cache_column] || 0) - 1 unless record.frozen?
 
                 if record.persisted?

spec/mongoid/association/fallback_spec.rb

@@ -378,6 +378,29 @@ class Symphony
     end
   end
 
+  context 'with a counter cache' do
+    before(:all) do
+      class Composer
+        include Mongoid::Document
+      end
+
+      class Symphony
+        include Mongoid::Document
+
+        belongs_to :composer, counter_cache: true, fallback: -> { Anonymous.new }
+      end
+    end
+
+    after(:all) do
+      Object.send(:remove_const, :Symphony)
+      Object.send(:remove_const, :Composer)
+    end
+
+    it 'does not update the counter on the fallback' do
+      expect { Symphony.create! }.not_to raise_error
+    end
+  end
+
   context 'validation of the :fallback option' do
     it 'rejects a declaration that combines :fallback with :autobuild' do
       expect do