[IMP] base_report_to_printer: Avoid writing on printing printers/jobs if no change

The cron (by default) every minute, updates the printing.printer and
printing.job records from the cups server. The field values rarely
change, so we can prevent many updates in database.

Also, writing on `printing_printer.server_id` triggers another update
on the printing.job records as the model has a stored related field.

base_report_to_printer/models/printing_printer.py

@@ -69,7 +69,7 @@ class PrintingPrinter(models.Model):
 
     def _prepare_update_from_cups(self, cups_connection, cups_printer):
         mapping = {3: "available", 4: "printing", 5: "error"}
-        vals = {
+        cups_vals = {
             "name": cups_printer["printer-info"],
             "model": cups_printer.get("printer-make-and-model", False),
             "location": cups_printer.get("printer-location", False),
@@ -77,6 +77,14 @@ class PrintingPrinter(models.Model):
             "status": mapping.get(cups_printer.get("printer-state"), "unknown"),
             "status_message": cups_printer.get("printer-state-message", ""),
         }
+
+        # prevent write if the field didn't change
+        vals = {
+            fieldname: value
+            for fieldname, value in cups_vals.items()
+            if not self or value != self[fieldname]
+        }
+
         printer_uri = cups_printer["printer-uri-supported"]
         printer_system_name = printer_uri[printer_uri.rfind("/") + 1 :]
         ppd_info = cups_connection.getPPD3(printer_system_name)
@@ -96,13 +104,13 @@ class PrintingPrinter(models.Model):
         if not option:
             return vals
 
-        vals["tray_ids"] = []
+        tray_commands = []
         cups_trays = {
             tray_option["choice"]: tray_option["text"] for tray_option in option.choices
         }
 
         # Add new trays
-        vals["tray_ids"].extend(
+        tray_commands.extend(
             [
                 (0, 0, {"name": text, "system_name": choice})
                 for choice, text in cups_trays.items()
@@ -111,7 +119,7 @@ class PrintingPrinter(models.Model):
         )
 
         # Remove deleted trays
-        vals["tray_ids"].extend(
+        tray_commands.extend(
             [
                 (2, tray.id)
                 for tray in self.tray_ids.filtered(
@@ -119,6 +127,8 @@ class PrintingPrinter(models.Model):
                 )
             ]
         )
+        if tray_commands:
+            vals["tray_ids"] = tray_commands
         return vals
 
     def print_document(self, report, content, **print_opts):