为什么SQLAlchemy不创建串行列？

Question

SQLAlchemy正在为postgresql中的列生成序列，但没有启用。我怀疑我可能是在发动机设置上做错了什么。

使用SQLAlchemy教程（http://docs.sqlalchemy.org/en/rel_0_9/orm/tutorial.html）中的示例：

#!/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)

使用此脚本，将生成下表：

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

但是，创建了一个序列：

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

-里斯

Answer 1

这是因为您为它提供了一个显式的Sequence。postgresql中的SERIAL数据类型生成它自己的序列，SQLAlchemy知道如何定位该序列-因此，如果省略Sequence，SQLAlchemy将呈现SERIAL，假设意图是列是自动递增的（由autoincrement参数和整数主关键字共同决定；它默认为True）。但是当Sequence被传递时，SQLAlchemy看到的意图是您不希望由SERIAL隐式创建序列，而是您指定的序列：

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)

输出：

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)
)

Answer 2

里斯

我还将该教程用作一个模型，只是无法让它与任何Postgres表一起使用，这些表已经存在，并且具有带有序列的键ID列，以生成新的键ID值。

和David一样，我发现序列必须单独定义给类。对于任何使用“db.Model”方法的人，这里有一个例子。

from flask.ext.sqlalchemy import SQLAlchemy
from sqlalchemy import Sequence
db = SQLAlchemy()

pageimpression_imp_id_seq = Sequence('pageimpression_imp_id_seq')
class PageImpression(db.Model):
        __tablename__ = 'pageimpression'
        imp_id = db.Column(db.Integer,     
    pageimpression_imp_id_seq,           
    server_default=usersession_sessionid_seq.next_value(),primary_key=True)
    logdate = db.Column(db.DateTime)
    sessionid = db.Column(db.String)
    path = db.Column(db.String)
    referrer = db.Column(db.String)

def __init__(self, imp_id, logdate, sessionid, path, referrer):
    self.imp_id = imp_id
    self.logdate = logdate
    self.sessionid = sessionid
    self.path = path
    self.referrer = referrer

def __repr__(self):
   return "<PageImpression(imp_id='%s', logdate='%s',sessionid='%s', path='%s', referrer='%s')>" % (self.imp_id, self.logdate, self.sessionid, self.path, self.referrer)

def PageImpressionAdd(sessionid):
    sessionid = 0 # dummy value for unit testing
    current_time = datetime.now().isoformat()
    if CurrentConfig.IMPRESSION_LOGGING_ON == True:     
        path = request.path
        if request.environ.get('HTTP_REFERER') and not request.environ.get('HTTP_REFERER').isspace():
            referrer = request.environ.get('HTTP_REFERER') # the string is not-empty
        else:
            referrer = '' # the string is empty
        from website.models import PageImpression
        thisPageImpression = PageImpression(None,current_time,sessionid, path, referrer)
        db.session.add(thisPageImpression)
        db.session.commit()
        # get the values created by the Postgres table defaults
        imp_id = thisPageImpression.imp_id
        logdate = thisPageImpression.logdate
    return current_time

Answer 3

如果由于某些原因需要显式地创建序列（如设置起始值），并且仍然需要与使用Column(Integer, primary_key=True)符号时相同的默认值行为，则可以使用以下代码完成该操作：

#!/usr/bin/env python

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

Base = declarative_base()
USER_ID_SEQ = Sequence('user_id_seq')  # define sequence explicitly
class User(Base):
    __tablename__ = 'users'
    # use sequence in column definition, and pass .next_value() as server_default
    id = Column(Integer, USER_ID_SEQ, primary_key=True, server_default=USER_ID_SEQ.next_value())
    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)