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Does the SQL Alchemy Relationship loader leave a lock on the table?

I have very simple code causing my MySQL db to freeze:

import sqlalchemy as sa
from sqlalchemy import orm

# creating the engine, the base, etc
import utils
import config

utils.base_init(config)
Base = config.Base

class Parent(Base):
    __tablename__ = 'Parents'
    id = sa.Column(sa.Integer, primary_key=True)
    children = orm.relationship('Child', backref='parent')

class Child(Base):
    id = sa.Column(sa.Integer, primary_key=True)
    parent_id = sa.Column(sa.Integer)

    __tablename__ = 'Children'

    __table_args__ = (sa.ForeignKeyConstraint(
        ['parent_id'],
        ['Parents.id'],
        onupdate='CASCADE', ondelete='CASCADE'),{})

Base.metadata.create_all()

session = orm.sessionmaker(bind=config.Base.metadata.bind)()
p = Parent(id=1)
c1 = Child(id=1)
c2 = Child(id=2)
session.add(p)
session.add(c1)
session.add(c2)
session.commit()

# Works
# Base.metadata.drop_all()

c1.parent
# 2012-08-17 20:16:21,459 INFO sqlalchemy.engine.base.Engine BEGIN (implicit)
# 2012-08-17 20:16:21,460 INFO sqlalchemy.engine.base.Engine SELECT `Children`.id AS `Children_id`, `Children`.parent_id AS `Children_parent_id` 
# FROM `Children` 
# WHERE `Children`.id = %s
# 2012-08-17 20:16:21,460 INFO sqlalchemy.engine.base.Engine (1,)



Base.metadata.drop_all()
# hangs until i kill the connection above.
# server status: 'Waiting for table metadata lock'

It seems that SQL Alchemy does not release the metadata lock after issuing the selection request needed to load the relation attribute? How can I free him? I don’t even understand why the select statement will have to lock the table first!

Of course, I can make this particular piece of code work by closing the session, but this is not practical in my real program.

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1 answer

You need to start a new transaction before calling .drop_all(); MySQL sees what you read from the table in this transaction and block the table from deleting it:

session.commit()
Base.metadata.drop_all()

.

MySQL ; , , , . DROP TABLE MySQL , .

, MySQL, . , ; isolation_level create_engine():

engine = create_engine(
    'mysql://username:passwd@localhost/databasename',
    isolation_level='READ UNCOMMITTED')

. SET TRANSACTION.

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