diff --git a/models/res_partner_contract.py b/models/res_partner_contract.py
index a821f95..1995432 100644
--- a/models/res_partner_contract.py
+++ b/models/res_partner_contract.py
@@ -20,6 +20,9 @@ class PartnerContract(models.Model):
         string="Annexes",
         help="Annexes to this contract",
     )
+    contract_annex_number = fields.Integer(
+        default=1, help="Counter for generate Annex name"
+    )
     partner_id = fields.Many2one(
         "res.partner",
         string="Partner",
diff --git a/models/res_partner_contract_annex.py b/models/res_partner_contract_annex.py
index 80e8ea3..d7770b9 100644
--- a/models/res_partner_contract_annex.py
+++ b/models/res_partner_contract_annex.py
@@ -8,30 +8,42 @@ class ContractOrderAnnex(models.Model):
     _description = "Contract Order Annex"
 
     name = fields.Char(string="Name", help="The Number of Annex")
-    order_id = fields.Many2one(
-        "sale.order",
-        string="Order",
-        domain=[("contract_annex_id", "=", False)],
-        required=True,
-    )
+    order_id = fields.Many2one("sale.order", string="Order", required=True,)
     contract_id = fields.Many2one(
-        "res.partner.contract", string="Contract", readonly=True
+        "res.partner.contract", string="Contract", readonly=True,
     )
 
+    @api.onchange("contract_id")
+    def _onchange_contract_id(self):
+        # Compute name if there is no custom name
+        contract_number = self.contract_id.name
+        annex_number = self.contract_id.contract_annex_number
+
+        self.name = "{contract}--{annex}".format(
+            contract=contract_number, annex=annex_number
+        )
+
+        # Compute domain for order_id because of bug with
+        # not working correctly domain in model
+        return {
+            "domain": {
+                "order_id": [
+                    ("partner_id", "=", self.contract_id.partner_id.id),
+                    ("contract_annex_id", "=", False),
+                ]
+            }
+        }
+
     @api.model
     def create(self, values):
         record = super().create(values)
 
-        # Compute name if there is no custom name
-        if not record.name:
-            contract_number = record.contract_id.name
-            annex_number = len(record.contract_id.contract_annex_ids.ids)
-
-            record.name = "{contract}--{annex}".format(
-                contract=contract_number, annex=annex_number
-            )
+        # Fill annex_id to domain it in future
         record.order_id.contract_annex_id = record.id
 
+        # Add counter
+        record.contract_id.contract_annex_number += 1
+
         return record
 
     @api.multi
diff --git a/views/res_partner_contract.xml b/views/res_partner_contract.xml
index cee51cd..6f871fa 100644
--- a/views/res_partner_contract.xml
+++ b/views/res_partner_contract.xml
@@ -36,6 +36,9 @@
                         <field name="date_conclusion" attrs="{'readonly': [('state', 'in', ['sign', 'close'])]}"/>
                         <field name="date_conclusion_fix"/>
                         <field name="partner_id" readonly="1"/>
+
+                        <!-- Uses to generate number of Annex -->
+                        <field name="contract_annex_number" invisible="1"/> 
                     </group>
                     <group string="Annexed orders" name="multi_params">
                         <field name="contract_annex_ids" attrs="{'readonly': [('state', 'in', ['sign', 'close'])]}">