Parameter validation via plugin (#4958)

* Expose part parameter validation to plugins

- Allow ValidationMixin plugins to validate part parameter values

* Update sample plugin

* Catch and re-throw error

* Update docs

* Improve plugin docs

* Simplify validation sample

* Calculate numeric value first

InvenTree/part/models.py

@@ -572,7 +572,7 @@ class Part(InvenTreeBarcodeMixin, InvenTreeNotesMixin, MetadataMixin, MPTTModel)
         """Ensure that the IPN (internal part number) is valid for this Part"
 
         - Validation is handled by custom plugins
-        - By default, no validation checks are perfomed
+        - By default, no validation checks are performed
         """
 
         from plugin.registry import registry
@@ -3505,6 +3505,24 @@ class PartParameter(MetadataMixin, models.Model):
                     'data': _('Invalid choice for parameter value')
                 })
 
+        self.calculate_numeric_value()
+
+        # Run custom validation checks (via plugins)
+        from plugin.registry import registry
+
+        for plugin in registry.with_mixin('validation'):
+
+            # Note: The validate_part_parameter function may raise a ValidationError
+            try:
+                result = plugin.validate_part_parameter(self, self.data)
+                if result:
+                    break
+            except ValidationError as exc:
+                # Re-throw the ValidationError against the 'data' field
+                raise ValidationError({
+                    'data': exc.message
+                })
+
     def calculate_numeric_value(self):
         """Calculate a numeric value for the parameter data.