Document rules and enforce price value uniqueness

modules/price/message.xml

@@ -3,6 +3,9 @@
 this repository contains the full copyright notices and license terms. -->
 <tryton>
     <data grouped="1">
+        <record model="ir.message" id="msg_price_value_unique">
+            <field name="text">Only one price value is allowed for each price index and price date.</field>
+        </record>
         <record model="ir.message" id="msg_party_code_unique">
             <field name="text">The code on party must be unique.</field>
         </record>

modules/price/price_value.py

@@ -9,7 +9,7 @@ from trytond.i18n import gettext
 from trytond.model import (
     DeactivableMixin, Index, ModelSQL, ModelView, MultiValueMixin, Unique,
     ValueMixin, convert_from, fields, sequence_ordered)
-from trytond.model.exceptions import AccessError
+from trytond.model.exceptions import AccessError, ValidationError
 from trytond.pool import Pool
 from trytond.pyson import Bool, Eval
 from trytond.tools import is_full_text, lstrip_wildcard
@@ -38,12 +38,34 @@ class PriceValue(
     price_value = fields.Float("Price value")
     open_price = fields.Float("Open price")
     low_price = fields.Float("Low price")
-    high_price = fields.Float("High price")
-
-    def get_price_index(self, name):
-        if self.price:
-            return self.price.price_index
-        return None
+    high_price = fields.Float("High price")
+
+    @classmethod
+    def validate(cls, price_values):
+        super().validate(price_values)
+        cls.check_unique_price_date(price_values)
+
+    @classmethod
+    def check_unique_price_date(cls, price_values):
+        domains = []
+        for price_value in price_values:
+            if not price_value.price or not price_value.price_date:
+                continue
+            domain = [
+                ('price', '=', price_value.price.id),
+                ('price_date', '=', price_value.price_date),
+                ]
+            if price_value.id:
+                domain.append(('id', '!=', price_value.id))
+            domains.append(['AND'] + domain)
+        if domains and cls.search(['OR'] + domains, limit=1):
+            raise ValidationError(
+                gettext('price.msg_price_value_unique'))
+
+    def get_price_index(self, name):
+        if self.price:
+            return self.price.price_index
+        return None
 
 class PriceValueReport(
         ModelSQL, ModelView):