KeyError for output_field in GeneratedField

[Paolo Melchiorre]

Trying to get SQL code for a migration I receive a KeyError.

Model

Example of a model with a GenratedField.

from decimal import Decimal
from django.db import models
from django.db.models import F, Value as V
from django.db.models.functions import Round

class Item(models.Model):
    price = models.DecimalField(max_digits=7, decimal_places=2)
    vat_price = models.GeneratedField(
        db_persist=True,
        expression=Round(F("price") * V(Decimal("1.22")), 2),
        output_field=models.DecimalField(max_digits=8, decimal_places=2),
    )

Step

Generate the migration file:

$ python -m manage makemigrations

Steps to generate the error:

$ python -m manage sqlmigrate shop 0001

Similar error with another command:

$ python -m manage migrate shop 0001

Traceback

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/home/paulox/Projects/generatedfield/manage.py", line 22, in <module>
    main()
  File "/home/paulox/Projects/generatedfield/manage.py", line 18, in main
    execute_from_command_line(sys.argv)
  File "/home/paulox/Projects/generatedfield/.venv/lib/python3.11/site-packages/django/core/management/__init__.py", line 442, in execute_from_command_line
    utility.execute()
  File "/home/paulox/Projects/generatedfield/.venv/lib/python3.11/site-packages/django/core/management/__init__.py", line 436, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/paulox/Projects/generatedfield/.venv/lib/python3.11/site-packages/django/core/management/base.py", line 412, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/home/paulox/Projects/generatedfield/.venv/lib/python3.11/site-packages/django/core/management/commands/sqlmigrate.py", line 38, in execute
    return super().execute(*args, **options)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/paulox/Projects/generatedfield/.venv/lib/python3.11/site-packages/django/core/management/base.py", line 458, in execute
    output = self.handle(*args, **options)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/paulox/Projects/generatedfield/.venv/lib/python3.11/site-packages/django/core/management/commands/sqlmigrate.py", line 80, in handle
    sql_statements = loader.collect_sql(plan)
                     ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/paulox/Projects/generatedfield/.venv/lib/python3.11/site-packages/django/db/migrations/loader.py", line 381, in collect_sql
    state = migration.apply(state, schema_editor, collect_sql=True)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/paulox/Projects/generatedfield/.venv/lib/python3.11/site-packages/django/db/migrations/migration.py", line 132, in apply
    operation.database_forwards(
  File "/home/paulox/Projects/generatedfield/.venv/lib/python3.11/site-packages/django/db/migrations/operations/models.py", line 96, in database_forwards
    schema_editor.create_model(model)
  File "/home/paulox/Projects/generatedfield/.venv/lib/python3.11/site-packages/django/db/backends/base/schema.py", line 506, in create_model
    self.deferred_sql.extend(self._model_indexes_sql(model))
                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/paulox/Projects/generatedfield/.venv/lib/python3.11/site-packages/django/db/backends/base/schema.py", line 1595, in _model_indexes_sql
    output.extend(self._field_indexes_sql(model, field))
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/paulox/Projects/generatedfield/.venv/lib/python3.11/site-packages/django/db/backends/postgresql/schema.py", line 63, in _field_indexes_sql
    like_index_statement = self._create_like_index_sql(model, field)
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/paulox/Projects/generatedfield/.venv/lib/python3.11/site-packages/django/db/backends/postgresql/schema.py", line 88, in _create_like_index_sql
    db_type = field.db_type(connection=self.connection)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/paulox/Projects/generatedfield/.venv/lib/python3.11/site-packages/django/db/models/fields/__init__.py", line 879, in db_type
    return column_type % data
           ~~~~~~~~~~~~^~~~~~
  File "/home/paulox/Projects/generatedfield/.venv/lib/python3.11/site-packages/django/utils/datastructures.py", line 280, in __getitem__
    value = super().__getitem__(key)
            ^^^^^^^^^^^^^^^^^^^^^^^^
KeyError: 'max_digits'

Expected result

BEGIN;
--
-- Create model Item
--
CREATE TABLE "shop_item" (
    "id" bigint NOT NULL PRIMARY KEY GENERATED BY DEFAULT AS IDENTITY,
    "price" numeric(7, 2) NOT NULL,
    "vat_price" numeric(8, 2) GENERATED ALWAYS AS (ROUND(("price" * 1.22), 2)) STORED
);
COMMIT;