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
Advertisement
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)
)