[UPD] store category on membership line

models/__init__.py

@@ -2,4 +2,5 @@
 # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
 
 from . import account_move
+from . import membership_line
 from . import res_partner

models/membership_line.py

+# Copyright 2021 Le Filament (<http://www.le-filament.com>)
+# License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
+
+from odoo import fields, models
+
+
+class MembershipLine(models.Model):
+    _inherit = "membership.membership_line"
+
+    category_id = fields.Many2one(store=True)