为什么 SQLAlchemy 不创建串行列？

Question

SQLAlchemy 正在为 postgresql 中的列生成但不启用序列。我怀疑我在引擎设置中可能做错了什么。

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，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

然而，一个序列被创建：

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

这是因为您为其提供了明确的Sequence。 postgresql 中的SERIAL 数据类型生成其自己的 序列，SQLAlchemy 知道如何定位 - 所以如果省略Sequence，SQLAlchemy 将呈现SERIAL，假设意图是该列是自动递增（由 autoincrement 参数与 Integer primary_key 一起确定；默认为 True）。但是当Sequence 被传递时，SQLAlchemy 看到您不希望由SERIAL 隐式创建的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

如果您出于某种原因需要显式创建序列，例如设置起始值，并且仍希望使用与使用 Column(Integer, primary_key=True) 表示法时相同的默认值行为，则可以使用以下代码完成：

#!/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)

Answer 3

里斯

我也将该教程用作模型，但无法让它与任何已经存在的 Postgres 表一起工作，并且具有带有序列序列的键 ID 列来生成新的键 ID 值。

和 David 一样，我发现必须将序列单独定义到类中。对于使用“db.Model”方法的任何人，这里有一个示例。

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 4

您也可以通过 GUI pgAdmin 更改序列而无需任何 SQL 脚本，如下所示：

选择您的数据库 -> 架构 -> 公共 -> 序列 -> 右键单击​​ -> 属性 -> 定义 -> 当前值。