Allow atomic set from explicit nil on array to new values. Fixes #1587

CHANGELOG.md

@@ -50,6 +50,9 @@ For instructions on upgrading to newer versions, visit
 * \#1589 Allow assignment of empty array to HABTM when no documents are yet
   loaded into memory.
 
+* \#1587 When a previous value for an array field was an explicit nil, it can
+  now be reset atomically with new values.
+
 * \#1585 `Model#respond_to?` returns true now for the setter when allowing
   dynamic fields.
 

lib/mongoid/dirty.rb

@@ -131,8 +131,7 @@ def setters
             field = fields[name]
             key = embedded? ? "#{atomic_position}.#{name}" : name
             if field && field.resizable?
-              pushes, pulls = (new || []) - (old || []), (old || []) - (new || [])
-              field.add_atomic_changes(self, key, modifications, pushes, pulls)
+              field.add_atomic_changes(self, key, modifications, new, old)
             else
               modifications[key] = new
             end

lib/mongoid/fields/internal/array.rb

@@ -20,12 +20,16 @@ class Array
         #
         # @since 2.4.0
         def add_atomic_changes(document, key, mods, new, old)
-          if !new.empty? && !old.empty?
+          pushes = (new || []) - (old || [])
+          pulls = (old || []) - (new || [])
+          if old.nil?
+            mods[key] = pushes
+          elsif !pushes.empty? && !pulls.empty?
             mods[key] = document.attributes[key]
-          elsif !new.empty?
-            document.atomic_array_pushes[key] = new
-          elsif !old.empty?
-            document.atomic_array_pulls[key] = old
+          elsif !pushes.empty?
+            document.atomic_array_pushes[key] = pushes
+          elsif !pulls.empty?
+            document.atomic_array_pulls[key] = pulls
           end
         end
 

lib/mongoid/fields/internal/foreign_keys/array.rb

@@ -21,12 +21,16 @@ class Array
           #
           # @since 2.4.0
           def add_atomic_changes(document, key, mods, new, old)
-            if new.any? && old.any?
+            pushes = (new || []) - (old || [])
+            pulls = (old || []) - (new || [])
+            if old.nil?
+              mods[key] = pushes
+            elsif !pushes.empty? && !pulls.empty?
               mods[key] = document.attributes[key]
-            elsif new.any?
-              document.atomic_array_add_to_sets[key] = new
-            elsif old.any?
-              document.atomic_array_pulls[key] = old
+            elsif !pushes.empty?
+              document.atomic_array_add_to_sets[key] = pushes
+            elsif !pulls.empty?
+              document.atomic_array_pulls[key] = pulls
             end
           end
 

spec/functional/mongoid/relations/embedded/one_spec.rb

@@ -643,4 +643,39 @@
       end
     end
   end
+
+  context "when the embedded document has an array field" do
+
+    let!(:person) do
+      Person.create(:ssn => "673-11-2321")
+    end
+
+    let!(:name) do
+      person.create_name(
+        :first_name => "Syd",
+        :last_name => "Vicious",
+        :aliases => nil
+      )
+    end
+
+    context "when saving the array on a persisted document" do
+
+      let(:from_db) do
+        Person.find(person.id).name
+      end
+
+      before do
+        from_db.aliases = [ "Syd", "Sydney" ]
+        from_db.save
+      end
+
+      it "sets the values of the array" do
+        from_db.aliases.should eq([ "Syd", "Sydney" ])
+      end
+
+      it "persists the array" do
+        Person.find(person.id).name.aliases.should eq([ "Syd", "Sydney" ])
+      end
+    end
+  end
 end