Do not update created_at on update (#324)

spec/granite/transactions/update_spec.cr

@@ -20,6 +20,27 @@ describe "#update" do
 
     Parent.find!(parent.id).name.should eq "New Parent"
   end
+
+  context "when created_at is nil" do
+    it "does not update created_at" do
+      parent = Parent.new(name: "New Parent")
+      parent.save!
+
+      created_at = parent.created_at!.at_beginning_of_second
+
+      # Simulating instantiating a new object with same ID
+      new_parent = Parent.new(name: "New New Parent")
+      new_parent.id = parent.id
+      new_parent.new_record = false
+      new_parent.updated_at = parent.updated_at
+      new_parent.save!
+
+      saved_parent = Parent.find!(parent.id)
+      saved_parent.name.should eq "New New Parent"
+      saved_parent.created_at.should eq created_at
+      saved_parent.updated_at.should eq Time.utc_now.at_beginning_of_second
+    end
+  end
 end
 
 describe "#update!" do

src/granite/transactions.cr

@@ -125,9 +125,15 @@ module Granite::Transactions
 
     private def __update
       set_timestamps mode: :update
-      fields = self.class.content_fields
+      fields = self.class.content_fields.dup
       params = content_values + [@{{primary_name}}]
 
+      # Do not update created_at on update
+      if created_at_index = fields.index("created_at")
+        fields.delete_at created_at_index
+        params.delete_at created_at_index
+      end
+
       begin
         @@adapter.update @@table_name, @@primary_name, fields, params
       rescue err
@@ -239,7 +245,7 @@ module Granite::Transactions
   # Returns true if this object hasn't been saved yet.
   @[JSON::Field(ignore: true)]
   @[YAML::Field(ignore: true)]
-  getter? new_record : Bool = true
+  property? new_record : Bool = true
 
   # Returns true if this object has been destroyed.
   @[JSON::Field(ignore: true)]