Sale.Order readonly boolean field

[Attila-Alz]

Hello,

I have a non stored, computed field "is_expired" which is also readonly by default when the model is generated. So I get the error "{Odoo Server Error, 'sale.order' object attribute 'is_expired' is read-only}".
Adding "readonly=True" in order.sale.py doesn't solve this problem, it does however change the summary text to ReadOnly: False when generating the model again.

...
            //var tableName = "sale.order";
            //var modelResult = await odooClient.GetModelAsync(tableName);
            //var model = OdooModelMapper.GetDotNetModel(tableName, modelResult.Value);

            var saleOrderRepository = new OdooRepository<SaleOrderOdooModel>(config);
            var saleOrders = await saleOrderRepository.Query()
                .Where(x=>x.OrderNumber, OdooOperator.EqualsTo, orderNumber)
                .ToListAsync();
...

C# Model (generated):

...
        /// <summary>
        /// is_expired - boolean  <br />
        /// Required: False, Readonly: True, Store: False, Sortable: False <br />
        /// </summary>
        [JsonProperty("is_expired")]
        public bool? IsExpired { get; set; }
...

Odoo sale_order.py file:

...
is_expired = fields.Boolean(string="Is Expired", compute='_compute_is_expired') **//Adding readonly=True doesn't solve the problem**
...
def _compute_is_expired(self):
    today = fields.Date.today()
    for order in self:
        order.is_expired = order.state == 'sent' and order.validity_date and order.validity_date < today
...

How can I make this work?