[FIX] contract: Currency is not editable + pricelist from partner

Previous related field was not accurated nor editable. Now the field is got
properly from a computed field.

Reviewing this, as the currency was taken (and it continues being taken) from the partner
pricelist if no pricelist is explicitly set, automatic price should use the same logic for
using partner pricelist.

contract/models/contract.py

@@ -33,7 +33,8 @@ class ContractContract(models.Model):
         ondelete='restrict',
     )
     currency_id = fields.Many2one(
-        related="company_id.currency_id",
+        compute="_compute_currency_id",
+        comodel_name="res.currency",
         string="Currency",
         readonly=True,
     )
@@ -147,6 +148,24 @@ class ContractContract(models.Model):
         )
         return invoices
 
+    @api.depends("pricelist_id", "partner_id", "journal_id", "company_id")
+    def _compute_currency_id(self):
+        for rec in self:
+            currency = self.env['res.currency']
+            if any(rec.contract_line_ids.mapped('automatic_price')):
+                # Use pricelist currency
+                currency = (
+                    rec.pricelist_id.currency_id or
+                    rec.partner_id.with_context(
+                        force_company=rec.company_id.id,
+                    ).property_product_pricelist.currency_id
+                )
+            rec.currency_id = (
+                currency.id or
+                rec.journal_id.currency_id.id or
+                rec.company_id.currency_id.id
+            )
+
     @api.multi
     def _compute_invoice_count(self):
         for rec in self:
@@ -299,11 +318,6 @@ class ContractContract(models.Model):
                 _("Please define a %s journal for the company '%s'.")
                 % (self.contract_type, self.company_id.name or '')
             )
-        currency = (
-            self.pricelist_id.currency_id
-            or self.partner_id.property_product_pricelist.currency_id
-            or self.company_id.currency_id
-        )
         invoice_type = 'out_invoice'
         if self.contract_type == 'purchase':
             invoice_type = 'in_invoice'
@@ -318,7 +332,7 @@ class ContractContract(models.Model):
         invoice_vals = vinvoice._convert_to_write(vinvoice._cache)
         invoice_vals.update({
             'name': self.code,
-            'currency_id': currency.id,
+            'currency_id': self.currency_id.id,
             'date_invoice': date_invoice,
             'journal_id': journal.id,
             'origin': self.name,