Traceback and sample code for pg_conn error

pgconn_traceback.py

Traceback (most recent call last):
  File "/app/manage.py", line 22, in <module>
    main()
  File "/app/manage.py", line 18, in main
    execute_from_command_line(sys.argv)
  File "/env/lib/python3.11/site-packages/django/core/management/__init__.py", line 442, in execute_from_command_line
    utility.execute()
  File "/env/lib/python3.11/site-packages/django/core/management/__init__.py", line 436, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/env/lib/python3.11/site-packages/django/core/management/base.py", line 412, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/env/lib/python3.11/site-packages/django/core/management/base.py", line 458, in execute
    output = self.handle(*args, **options)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/core/management/commands/populate_search_vectors.py", line 48, in handle
    Model.objects.build_search_vector_annotations().update(
  File "/env/lib/python3.11/site-packages/django/db/models/query.py", line 1206, in update
    rows = query.get_compiler(self.db).execute_sql(CURSOR)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/env/lib/python3.11/site-packages/django/db/models/sql/compiler.py", line 1982, in execute_sql
    cursor = super().execute_sql(result_type)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/env/lib/python3.11/site-packages/django/db/models/sql/compiler.py", line 1547, in execute_sql
    sql, params = self.as_sql()
                  ^^^^^^^^^^^^^
  File "/env/lib/python3.11/site-packages/django/db/models/sql/compiler.py", line 1954, in as_sql
    sql, params = self.compile(val)
                  ^^^^^^^^^^^^^^^^^
  File "/env/lib/python3.11/site-packages/django/db/models/sql/compiler.py", line 544, in compile
    sql, params = node.as_sql(self, self.connection)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/env/lib/python3.11/site-packages/django/db/models/expressions.py", line 680, in as_sql
    sql, params = compiler.compile(self.lhs)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/env/lib/python3.11/site-packages/django/db/models/sql/compiler.py", line 544, in compile
    sql, params = node.as_sql(self, self.connection)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/env/lib/python3.11/site-packages/django/contrib/postgres/search.py", line 149, in as_sql
    sql = connection.ops.compose_sql(sql, config_params + params + extra_params)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/env/lib/python3.11/site-packages/django/db/backends/postgresql/operations.py", line 205, in compose_sql
    return mogrify(sql, params, self.connection)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/env/lib/python3.11/site-packages/django/db/backends/postgresql/psycopg_any.py", line 21, in mogrify
    return ClientCursor(connection.connection).mogrify(sql, params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/env/lib/python3.11/site-packages/psycopg/cursor.py", line 672, in __init__
    super().__init__(connection)
  File "/env/lib/python3.11/site-packages/psycopg/cursor.py", line 65, in __init__
    self._pgconn = connection.pgconn
                   ^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'pgconn'

sample.py

class State(models.Model):
    """A model representing a state."""

    abbreviation = models.CharField(max_length=2)
    name = models.CharField(max_length=254)
      
      
class City(models.Model):
    """
    A model representing a city
    """

    name = models.CharField(max_length=254)
    search_vector = SearchVectorField(null=True)
    state = models.ForeignKey("places.State", on_delete=models.PROTECT)

    class Meta:
        indexes = [GinIndex(fields=["search_vector"])]

    @classmethod
    def build_search_vector(cls):
        return (
            SearchVector("name", weight="B")
            + SearchVector("state_name", weight="A")
        )
      
      
City.objects.annotate(
    state_name=Subquery(
        State.objects.filter(id=OuterRef("state_id")).values_list("name")[:1]
    )
).update(
    search_vector=Model.build_search_vector()
)