Why isn’t SQLAlchemy creating serial columns?

SQLAlchemy is generating, but not enabling, sequences for columns in postgresql. I suspect I may be doing something wrong in engine setup.

Using an example from the SQLAlchemy tutorial (http://docs.sqlalchemy.org/en/rel_0_9/orm/tutorial.html):

#!/usr/bin/env python

from sqlalchemy import create_engine, Column, Integer, String, Sequence
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()
class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, Sequence('user_id_seq'), primary_key=True)
    name = Column(String(50))
    fullname = Column(String(50))
    password = Column(String(12))

    def __repr__(self):
        return "<User(name='%s', fullname='%s', password='%s')>" % (
                                self.name, self.fullname, self.password)

db_url = 'postgresql://localhost/serial'
engine = create_engine(db_url, echo=True)
Base.metadata.create_all(engine)

With this script, the following table is generated:

serial=# d+ users
                                 Table "public.users"
  Column  |         Type          | Modifiers | Storage  | Stats target | Description 
----------+-----------------------+-----------+----------+--------------+-------------
 id       | integer               | not null  | plain    |              | 
 name     | character varying(50) |           | extended |              | 
 fullname | character varying(50) |           | extended |              | 
 password | character varying(12) |           | extended |              | 
Indexes:
    "users_pkey" PRIMARY KEY, btree (id)
Has OIDs: no

However, a sequence was created:

serial=# select sequence_schema,sequence_name,data_type from information_schema.sequences ;
 sequence_schema | sequence_name | data_type 
-----------------+---------------+-----------
 public          | user_id_seq   | bigint

SQLAlchemy 0.9.1, Python 2.7.5+, Postgresql 9.3.1, Ubuntu 13.10

-Reece

Answer

this is because you provided it with an explicit Sequence. The SERIAL datatype in postgresql generates its own sequence, which SQLAlchemy knows how to locate – so if you omit the Sequence, SQLAlchemy will render SERIAL, assuming the intent is that the column is auto-incrementing (which is determined by the autoincrement argument in conjunction with Integer primary_key; it defaults to True). But when Sequence is passed, SQLAlchemy sees the intent that you don’t want the sequence implicitly created by SERIAL but instead the one you are specifying:

from sqlalchemy import create_engine, Column, Integer, String, Sequence
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class T1(Base):
    __tablename__ = 't1'

    # emits CREATE SEQUENCE + INTEGER
    id = Column(Integer, Sequence('user_id_seq'), primary_key=True)

class T2(Base):
    __tablename__ = 't2'

    # emits SERIAL
    id = Column(Integer, primary_key=True)

class T3(Base):
    __tablename__ = 't3'

    # emits INTEGER
    id = Column(Integer, autoincrement=False, primary_key=True)

engine = create_engine("postgresql://scott:tiger@localhost/test", echo=True)
Base.metadata.create_all(engine)

output:

CREATE SEQUENCE user_id_seq

CREATE TABLE t1 (
    id INTEGER NOT NULL, 
    PRIMARY KEY (id)
)


CREATE TABLE t2 (
    id SERIAL NOT NULL, 
    PRIMARY KEY (id)
)


CREATE TABLE t3 (
    id INTEGER NOT NULL, 
    PRIMARY KEY (id)
)