Make ExcludedCol work with fields and use them when constructing SET clause

The recent changes to add support for custom update values had a side
effect that upserts with PostGIS related fields would break.

They would break while building the `SET` clause. Django would try to
figure out the right placeholder for the expression, even though
none is required. Since there was not associated field information,
it couldn't figure it out.

By passing the field information, we ensure we always build the SET
clause correctly.

Author: Photonios

psqlextra/expressions.py

@@ -1,4 +1,6 @@
-from django.db.models import CharField, expressions
+from typing import Union
+
+from django.db.models import CharField, Field, expressions
 
 
 class HStoreValue(expressions.Expression):
@@ -215,8 +217,18 @@ class ExcludedCol(expressions.Expression):
     See: https://www.postgresql.org/docs/current/sql-insert.html#SQL-ON-CONFLICT
     """
 
-    def __init__(self, name: str):
-        self.name = name
+    def __init__(self, field_or_name: Union[Field, str]):
+
+        # We support both field classes or just field names here. We prefer
+        # fields because when the expression is compiled, it might need
+        # the field information to figure out the correct placeholder.
+        # Even though that isn't require for this particular expression.
+        if isinstance(field_or_name, Field):
+            super().__init__(field_or_name)
+            self.name = field_or_name.column
+        else:
+            super().__init__(None)
+            self.name = field_or_name
 
     def as_sql(self, compiler, connection):
         quoted_name = connection.ops.quote_name(self.name)

psqlextra/query.py

@@ -591,7 +591,7 @@ def _get_upsert_fields(self, kwargs):
             has_default = field.default != NOT_PROVIDED
             if field.name in kwargs or field.column in kwargs:
                 insert_fields.append(field)
-                update_values[field.name] = ExcludedCol(field.column)
+                update_values[field.name] = ExcludedCol(field)
                 continue
             elif has_default:
                 insert_fields.append(field)
@@ -602,13 +602,13 @@ def _get_upsert_fields(self, kwargs):
             # instead of a concrete field, we have to handle that
             if field.primary_key is True and "pk" in kwargs:
                 insert_fields.append(field)
-                update_values[field.name] = ExcludedCol(field.column)
+                update_values[field.name] = ExcludedCol(field)
                 continue
 
             if self._is_magical_field(model_instance, field, is_insert=True):
                 insert_fields.append(field)
 
             if self._is_magical_field(model_instance, field, is_insert=False):
-                update_values[field.name] = ExcludedCol(field.column)
+                update_values[field.name] = ExcludedCol(field)
 
         return insert_fields, update_values