Mongoize is not called on update_all, when $set operator is used

lib/mongoid/atomic_update_preparer.rb

@@ -25,7 +25,7 @@ def prepare(attributes, klass)
           if key.to_s.start_with?('$')
             (atomic_updates[key] ||= {}).update(prepare_operation(klass, key, value))
           else
-            (atomic_updates['$set'] ||= {})[key] = mongoize_for(key, klass, key, value)
+            (atomic_updates['$set'] ||= {})[key] = mongoize_for('$set', klass, key, value)
           end
         end
       end
@@ -43,7 +43,7 @@ def prepare(attributes, klass)
       def prepare_operation(klass, key, value)
         value.each_with_object({}) do |(key2, value2), hash|
           key2 = klass.database_field_name(key2)
-          hash[key2] = value_for(key, klass, value2)
+          hash[key2] = value_for(key, klass, key2, value2)
         end
       end
 
@@ -53,14 +53,15 @@ def prepare_operation(klass, key, value)
       #
       # @param [ String ] operator The operator.
       # @param [ Class ] klass The model class.
+      # @param [ String | Symbol ] key The field key.
       # @param [ Object ] value The original value.
       #
       # @return [ Object ] Value prepared for the provided operator.
-      def value_for(operator, klass, value)
+      def value_for(operator, klass, key, value)
         case operator
         when '$rename' then value.to_s
         when '$addToSet', '$push' then value.mongoize
-        else mongoize_for(operator, klass, operator, value)
+        else mongoize_for(operator, klass, key, value)
         end
       end
 

spec/mongoid/contextual/mongo_spec.rb

@@ -3659,15 +3659,15 @@
         context "when the attributes must be mongoized" do
 
           before do
-            context.update_all("$set" => { member_count: "1" })
+            context.update_all("$set" => { location: LatLng.new(52.30, 13.25) })
           end
 
           it "updates the first matching document" do
-            expect(depeche_mode.reload.member_count).to eq(1)
+            expect(depeche_mode.reload.location).to eq(LatLng.new(52.30, 13.25))
           end
 
           it "updates the last matching document" do
-            expect(new_order.reload.member_count).to eq(1)
+            expect(new_order.reload.location).to eq(LatLng.new(52.30, 13.25))
           end
         end
       end

spec/support/models/band.rb

@@ -26,6 +26,7 @@ class Band
   field :mojo, type: Object
   field :tags, type: Hash
   field :fans
+  field :location, type: LatLng
 
   alias_attribute :d, :deleted
 

spec/support/models/lat_lng.rb

@@ -5,6 +5,8 @@ class LatLng
   attr_accessor :lat, :lng
 
   def self.demongoize(object)
+    return if object.nil?
+
     LatLng.new(object[1], object[0])
   end
 
@@ -15,4 +17,8 @@ def initialize(lat, lng)
   def mongoize
     [ lng, lat ]
   end
+
+  def ==(other)
+    lat == other.lat && lng == other.lng
+  end
 end