Fix 'symbol' field for CustomUnit (#7557)

* Fix 'symbol' field for CustomUnit

- Do not require 'symbol' to be unique

* Run check as part of validate_unique

src/backend/InvenTree/common/migrations/0027_alter_customunit_symbol.py

@@ -0,0 +1,18 @@
+# Generated by Django 4.2.12 on 2024-07-04 10:32
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('common', '0026_auto_20240608_1238'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='customunit',
+            name='symbol',
+            field=models.CharField(blank=True, help_text='Optional unit symbol', max_length=10, verbose_name='Symbol'),
+        ),
+    ]

src/backend/InvenTree/common/models.py

@@ -14,7 +14,7 @@ from datetime import timedelta, timezone
 from enum import Enum
 from io import BytesIO
 from secrets import compare_digest
-from typing import Any, Callable, TypedDict, Union
+from typing import Any, Callable, Collection, TypedDict, Union
 
 from django.apps import apps
 from django.conf import settings as django_settings
@@ -3042,6 +3042,18 @@ class CustomUnit(models.Model):
 
         return fmt
 
+    def validate_unique(self, exclude=None) -> None:
+        """Ensure that the custom unit is unique."""
+        super().validate_unique(exclude)
+
+        if self.symbol:
+            if (
+                CustomUnit.objects.filter(symbol=self.symbol)
+                .exclude(pk=self.pk)
+                .exists()
+            ):
+                raise ValidationError({'symbol': _('Unit symbol must be unique')})
+
     def clean(self):
         """Validate that the provided custom unit is indeed valid."""
         super().clean()
@@ -3083,7 +3095,6 @@ class CustomUnit(models.Model):
         max_length=10,
         verbose_name=_('Symbol'),
         help_text=_('Optional unit symbol'),
-        unique=True,
         blank=True,
     )