Only toggle deltas if there have been changes.

Or if the record’s a new one. Prompted by #1027.

lib/thinking_sphinx/active_record/callbacks/delta_callbacks.rb

@@ -4,9 +4,7 @@ class ThinkingSphinx::ActiveRecord::Callbacks::DeltaCallbacks <
   callbacks :after_commit, :before_save
 
   def after_commit
-    return unless !suspended? && delta_indices? && processors.any? { |processor|
-      processor.toggled?(instance)
-    }
+    return unless !suspended? && delta_indices? && toggled?
 
     delta_indices.each do |index|
       index.delta_processor.index index
@@ -18,7 +16,9 @@ def after_commit
   end
 
   def before_save
-    return unless !ThinkingSphinx::Callbacks.suspended? && delta_indices?
+    return unless new_or_changed?           &&
+      !ThinkingSphinx::Callbacks.suspended? &&
+      delta_indices?
 
     processors.each { |processor| processor.toggle instance }
   end
@@ -45,11 +45,19 @@ def indices
     @indices ||= config.index_set_class.new :classes => [instance.class]
   end
 
+  def new_or_changed?
+    instance.new_record? || instance.changed?
+  end
+
   def processors
     delta_indices.collect &:delta_processor
   end
 
   def suspended?
     ThinkingSphinx::Callbacks.suspended? || ThinkingSphinx::Deltas.suspended?
   end
+
+  def toggled?
+    processors.any? { |processor| processor.toggled?(instance) }
+  end
 end

spec/thinking_sphinx/active_record/callbacks/delta_callbacks_spec.rb

@@ -130,6 +130,10 @@
 
     before :each do
       allow(config).to receive_messages :index_set_class => double(:new => [index])
+      allow(instance).to receive_messages(
+        :changed?    => true,
+        :new_record? => false
+      )
     end
 
     it "sets delta to true if there are delta indices" do
@@ -145,5 +149,24 @@
 
       callbacks.before_save
     end
+
+    it "does not try to set delta to true if the instance is unchanged" do
+      allow(instance).to receive_messages :changed? => false
+
+      expect(processor).not_to receive(:toggle)
+
+      callbacks.before_save
+    end
+
+    it "does set delta to true if the instance is unchanged but new" do
+      allow(instance).to receive_messages(
+        :changed?    => false,
+        :new_record? => true
+      )
+
+      expect(processor).to receive(:toggle)
+
+      callbacks.before_save
+    end
   end
 end