[FIX] product_label: Correctly define label_ids on product.template, …

…like in odoo core

product_label/models/product_template.py

@@ -4,15 +4,34 @@
 # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
 
 
-from odoo import fields, models
+from odoo import api, fields, models
 
 
 class ProductTemplate(models.Model):
     _inherit = "product.template"
 
     label_ids = fields.Many2many(
         comodel_name="product.label",
-        related="product_variant_ids.label_ids",
+        compute="_compute_label_ids",
+        inverse="_inverse_label_ids",
         string="Labels",
         readonly=False,
     )
+
+    @api.depends("product_variant_ids", "product_variant_ids.label_ids")
+    def _compute_label_ids(self):
+        for p in self:
+            if len(p.product_variant_ids) == 1:
+                p.label_ids = p.product_variant_ids.label_ids
+            else:
+                p.label_ids = False
+
+    def _inverse_label_ids(self):
+        for p in self:
+            if len(p.product_variant_ids) == 1:
+                p.product_variant_ids.label_ids = p.label_ids
+
+    def _get_related_fields_variant_template(self):
+        res = super()._get_related_fields_variant_template()
+        res.append("label_ids")
+        return res