How can I configure a sequence object for my primary key column in sqlalchemy?

Question

I want MediaFolder class to have auto-generated id when adding to the database or initializing. I've tried:

from sqlalchemy import Column, String, BigInteger, ForeignKey, Sequence
from __init__ import Base


class MediaFolder(Base):
    __tablename__ = 'media_folder'

    seq = Sequence('id', start=20000000)
    id = Column(BigInteger, server_default=seq.next_value(), primary_key=True)

    name = Column(String, nullable=False)

    username = Column(String, nullable=False)

x = MediaFolder()
x.name = 'test seq'
x.username = 'test seq u'


from __init__ import db_session
db_session.add(x)
db_session.commit()

but when execute, I get:

Traceback (most recent call last):
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\engine\base.py", line 1249, in _execute_context
    cursor, statement, parameters, context
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\engine\default.py", line 580, in do_execute
    cursor.execute(statement, parameters)
psycopg2.errors.NotNullViolation: null value in column "id" violates not-null constraint
DETAIL:  Failing row contains (null, test seq, test seq u, null).


The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "C:/Users/Krzysiek/Desktop/praca/media_library/media/media_folder.py", line 29, in <module>
    db_session.commit()
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\orm\scoping.py", line 162, in do
    return getattr(self.registry(), name)(*args, **kwargs)
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\orm\session.py", line 1027, in commit
    self.transaction.commit()
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\orm\session.py", line 494, in commit
    self._prepare_impl()
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\orm\session.py", line 473, in _prepare_impl
    self.session.flush()
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\orm\session.py", line 2470, in flush
    self._flush(objects)
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\orm\session.py", line 2608, in _flush
    transaction.rollback(_capture_exception=True)
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\util\langhelpers.py", line 68, in __exit__
    compat.reraise(exc_type, exc_value, exc_tb)
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\util\compat.py", line 153, in reraise
    raise value
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\orm\session.py", line 2568, in _flush
    flush_context.execute()
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\orm\unitofwork.py", line 422, in execute
    rec.execute(self)
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\orm\unitofwork.py", line 589, in execute
    uow,
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\orm\persistence.py", line 245, in save_obj
    insert,
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\orm\persistence.py", line 1137, in _emit_insert_statements
    statement, params
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\engine\base.py", line 988, in execute
    return meth(self, multiparams, params)
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\sql\elements.py", line 287, in _execute_on_connection
    return connection._execute_clauseelement(self, multiparams, params)
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\engine\base.py", line 1107, in _execute_clauseelement
    distilled_params,
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\engine\base.py", line 1253, in _execute_context
    e, statement, parameters, cursor, context
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\engine\base.py", line 1473, in _handle_dbapi_exception
    util.raise_from_cause(sqlalchemy_exception, exc_info)
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\util\compat.py", line 398, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb, cause=cause)
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\util\compat.py", line 152, in reraise
    raise value.with_traceback(tb)
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\engine\base.py", line 1249, in _execute_context
    cursor, statement, parameters, context
  File "C:\Users\Krzysiek\Anaconda3\lib\site-packages\sqlalchemy\engine\default.py", line 580, in do_execute
    cursor.execute(statement, parameters)
sqlalchemy.exc.IntegrityError: (psycopg2.errors.NotNullViolation) null value in column "id" violates not-null constraint
DETAIL:  Failing row contains (null, test seq, test seq u, null).

[SQL: INSERT INTO media_folder (name, username) VALUES (%(name)s, %(username)s) RETURNING media_folder.id]
[parameters: {'name': 'test seq', 'username': 'test seq u'}]
(Background on this error at: http://sqlalche.me/e/gkpj)

Basically id returns null. I've tried everything to solve this or to understand the sequence. How can I have the id value generated properly? Or maybe is there another way to auto-generate values in sqlalchemy?

Answer 1

You can create the sequence as part of the id column declaration.

class MediaFolder(Base):
    __tablename__ = 'media_folder'

    seq = Sequence('id', start=20000000)
    id = Column(BigInteger, Sequence('media_folder_id', start=20000000), primary_key=True)

    name = Column(String, nullable=False)
    username = Column(String, nullable=False)

This is what is in the database after the code is executed:

test=# \d media_folder_id
                        Sequence "public.media_folder_id"
  Type  |  Start   | Minimum |       Maximum       | Increment | Cycles? | Cache 
--------+----------+---------+---------------------+-----------+---------+-------
 bigint | 20000000 |       1 | 9223372036854775807 |         1 | no      |     1

test=# SELECT * FROM media_folder;
    id    |   name   |  username  
----------+----------+------------
 20000000 | test seq | test seq u

(it's a good idea to give the sequence a more unique name than just "id").