我有一个简单的数据库,将附件存储为blob。
CREATE TABLE public.attachment
(
id integer NOT NULL,
attachdata oid,
CONSTRAINT attachment_pkey PRIMARY KEY (id)
)
-- Import a file
INSERT INTO attachment (id, attachdata) VALUES (1, lo_import('C:\\temp\blob_import.txt'))
-- Export back as file.
SELECT lo_export(attachdata, 'C:\temp\blob_export_postgres.txt') FROM attachment WHERE id = 1
我可以直接使用psycopg2读回此文件。
from psycopg2 import connect
con = connect(dbname="blobtest", user="postgres", password="postgres", host="localhost")
cur = con.cursor()
cur.execute("SELECT attachdata FROM attachment WHERE id = 1")
oid = cur.fetchone()[0]
obj = con.lobject(oid)
obj.export('C:\\temp\\blob_export_psycopg.txt')
当我使用sqlalchemy尝试相同操作时,attachdata是零的字节串。 我已经使用BLOB,LargeBinary和BINARY等类型测试了以下代码。 attachdata bytstring的大小似乎是OID值。
from sqlalchemy import create_engine
from sqlalchemy import Column, Integer, Binary
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker
Base = declarative_base()
Session = sessionmaker()
engine = create_engine('postgresql://postgres:postgres@localhost:5432/blobtest', echo=True)
Base.metadata.create_all(engine)
Session.configure(bind=engine)
class Attachment(Base):
__tablename__ ="attachment"
id = Column(Integer, primary_key=True)
attachdata = Column(Binary)
session = Session()
attachment = session.query(Attachment).get(1)
with open('C:\\temp\\blob_export_sqlalchemy.txt', 'wb') as f:
f.write(attachment.attachdata)
我已经搜索了sqlalchemy文档和各种资源,找不到如何使用sqlalchemy导出二进制数据的解决方案。
答案 0 :(得分:2)
我有同样的问题。似乎没有办法通过ORM获取大对象数据。因此,我将ORM和psycopg2引擎组合如下:
from sqlalchemy import create_engine
from sqlalchemy import Column, Integer
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker, scoped_session
from sqlalchemy.dialects.postgresql import OID
Base = declarative_base()
session_factory = sessionmaker()
engine = create_engine('postgresql+psycopg2://postgres:postgres@localhost:5432/postgres', echo=True)
Base.metadata.create_all(engine)
session_factory.configure(bind=engine)
Session = scoped_session(session_factory)
class Attachment(Base):
__tablename__ ="attachment"
id = Column(Integer, primary_key=True)
oid = Column(OID)
@classmethod
def insert_file(cls, filename):
conn = engine.raw_connection()
l_obj = conn.lobject(0, 'wb', 0)
with open(filename, 'rb') as f:
l_obj.write(f.read())
conn.commit()
conn.close()
session = Session()
attachment = cls(oid=l_obj.oid)
session.add(attachment)
session.commit()
return attachment.id
@classmethod
def get_file(cls, attachment_id, filename):
session = Session()
attachment = session.query(Attachment).get(attachment_id)
conn = engine.raw_connection()
l_obj = conn.lobject(attachment.oid, 'rb')
with open(filename, 'wb') as f:
f.write(l_obj.read())
conn.close()
if __name__ == '__main__':
my_id = Attachment.insert_file(r'C:\path\to\file')
Attachment.get_file(my_id, r'C:\path\to\file_out')
不太优雅,但似乎可以使用。
更新:
我现在正在使用事件
from sqlalchemy import create_engine, event
from sqlalchemy import Column, Integer
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker, scoped_session
from sqlalchemy.dialects.postgresql import OID
Base = declarative_base()
session_factory = sessionmaker()
engine = create_engine('postgresql+psycopg2://postgres:postgres@localhost:5432/postgres', echo=True)
Base.metadata.create_all(engine)
session_factory.configure(bind=engine)
Session = scoped_session(session_factory)
class Data(Base):
__tablename__ = "attachment"
id = Column(Integer, primary_key=True)
oid = Column(OID)
@event.listens_for(Data, 'after_delete')
def remove_large_object_after_delete(_, connection, target):
raw_connection = connection.connection
l_obj = raw_connection.lobject(target.oid, 'n')
l_obj.unlink()
raw_connection.commit()
@event.listens_for(Data, 'before_insert')
def add_large_object_before_insert(_, connection, target):
raw_connection = connection.connection
l_obj = raw_connection.lobject(0, 'wb', 0)
target.oid = l_obj.oid
l_obj.write(target.ldata)
raw_connection.commit()
@event.listens_for(Data, 'load')
def inject_large_object_after_load(target, _):
session = object_session(target)
conn = session.get_bind().raw_connection()
l_obj = conn.lobject(target.oid, 'rb')
target.ldata = l_obj.read()
if __name__ == '__main__':
session = Session()
# Put
data = Data()
data.ldata = 'your large data'
session.add(data)
session.commit()
id = data.id
# Get
data2 = session.query(Data).get(id)
print(data.ldata) # Your large data is here
# Delete
session.delete(data)
session.delete(data2)
session.commit()
session.flush()
session.close()
到目前为止效果很好。
我不明白为什么这些天Postgres大对象被忽略了。我大量使用它们。或说我想,但是这尤其具有挑战性。...