id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
34877	KeyError for output_field in GeneratedField	Paolo Melchiorre	nobody	"Trying to get SQL code for a migration I receive a `KeyError`.

**Model**

Example of a model with a `GenratedField`.

{{{#!python
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:

{{{#!bash
$ python -m manage makemigrations
}}}

Steps to generate the error:

{{{#!bash
$ python -m manage sqlmigrate shop 0001
}}}

Similar error with another command:

{{{#!bash
$ python -m manage migrate shop 0001
}}}

**Traceback**

{{{#!pycon
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**

{{{#!sql
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;
}}}
"	Bug	new	Database layer (models, ORM)	5.0	Release blocker		field, database, generated, output_field		Unreviewed	0	0	0	0	0	0