How to delete a table in SQLAlchemy?

问题

I want to delete a table using SQLAlchemy.

Since I am testing over and over again, I want to delete the table my_users so that I can start from scratch every single time.

So far I am using SQLAlchemy to execute raw SQL through the engine.execute() method:

sql = text('DROP TABLE IF EXISTS my_users;')
result = engine.execute(sql)

However, I wonder if there is some standard way to do so. The only one I could find is drop_all(), but it deletes all the structure, not only one specific table:

Base.metadata.drop_all(engine)   # all tables are deleted

For example, given this very basic example. It consists on a SQLite infrastructure with a single table my_users in which I add some content.

from sqlalchemy import create_engine, Column, Integer, String, text
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base

engine = create_engine('sqlite://', echo=False)
Base = declarative_base()

class User(Base):
    __tablename__ = "my_users"

    id = Column(Integer, primary_key=True)
    name = Column(String)

    def __init__(self, name):
        self.name = name

# Create all the tables in the database which are
# defined by Base's subclasses such as User
Base.metadata.create_all(engine)

# Construct a sessionmaker factory object
session = sessionmaker()

# Bind the sessionmaker to engine
session.configure(bind=engine)

# Generate a session to work with
s = session()

# Add some content
s.add(User('myname'))
s.commit()

# Fetch the data
print(s.query(User).filter(User.name == 'myname').one().name)

For this specific case, drop_all() would work, but it won't be convenient from the moment I start having more than one table and I want to keep the other ones.

回答1:

Just call drop() against the table object. From the docs:

Issue a DROP statement for this Table, using the given Connectable for connectivity.

In your case it should be:

User.__table__.drop()

If you get an exception like:

sqlalchemy.exc.UnboundExecutionError: Table object 'my_users' is not bound to an Engine or Connection. Execution can not proceed without a database to execute against

You need to pass the engine:

User.__table__.drop(engine)

回答2:

Alternative to calling cls.__table__.drop(your_engine), you can try this:

Base.metadata.drop_all(bind=your_engine, tables=[User.__table__])

This method as well as the create_all() method accept an optional argument tables, which takes an iterator of sqlalchemy.sql.schema.Table instances.

You can control which tables are to be created or dropped in this way.

回答3:

Below is example code you can execute in iPython to test the creation and deletion of a table on Postgres

from sqlalchemy import * # imports all needed modules from sqlalchemy

engine = create_engine('postgresql://python:python@127.0.0.1/production') # connection properties stored

metadata = MetaData() # stores the 'production' database's metadata

users = Table('users', metadata,
Column('user_id', Integer),
Column('first_name', String(150)),
Column('last_name', String(150)),
Column('email', String(255)),
schema='python') # defines the 'users' table structure in the 'python' schema of our connection to the 'production' db

users.create(engine) # creates the users table

users.drop(engine) # drops the users table

You can also preview my article on Wordpress with this same example and screenshots: oscarvalles.wordpress.com (search for SQL Alchemy).

回答4:

For the special case when you don't have access to the table class and just need to delete the table by table name you can use this code

import logging
from sqlalchemy import MetaData
from sqlalchemy import create_engine
from sqlalchemy.engine.url import URL
from sqlalchemy.ext.declarative import declarative_base

DATABASE = {
   'drivername': 'sqlite',
   # 'host': 'localhost',
   # 'port': '5432',
   # 'username': 'YOUR_USERNAME',
   # 'password': 'YOUR_PASSWORD',
   'database': '/path/to/your_db.sqlite'
}

def drop_table(table_name):
   engine = create_engine(URL(**DATABASE))
   base = declarative_base()
   metadata = MetaData(engine, reflect=True)
   table = metadata.tables.get(table_name)
   if table is not None:
       logging.info(f'Deleting {table_name} table')
       base.metadata.drop_all(engine, [table], checkfirst=True)

drop_table('users')

来源：https://stackoverflow.com/questions/35918605/how-to-delete-a-table-in-sqlalchemy

标签

python

sqlite

sqlalchemy

drop-table