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Build a Simple CRUD App with TiDB and SQLAlchemy
Learn how to build a simple CRUD application with TiDB and SQLAlchemy.
/tidb/dev/dev-guide-outdated-for-sqlalchemy

Build a Simple CRUD App with TiDB and SQLAlchemy

SQLAlchemy is a popular open-source ORM library for Python.

This document describes how to use TiDB and SQLAlchemy to build a simple CRUD application.

Note:

It is recommended to use Python 3.10 or a later Python version.

Step 1. Launch your TiDB cluster

The following introduces how to start a TiDB cluster.

Use a TiDB Serverless cluster

For detailed steps, see Create a TiDB Serverless cluster.

Use a local cluster

For detailed steps, see Deploy a local test cluster or Deploy a TiDB cluster using TiUP.

See Create a TiDB Serverless cluster.

Step 2. Get the code

git clone https://github.com/pingcap-inc/tidb-example-python.git

The following uses SQLAlchemy 1.44 as an example.

import uuid
from typing import List

from sqlalchemy import create_engine, String, Column, Integer, select, func
from sqlalchemy.orm import declarative_base, sessionmaker

engine = create_engine('mysql://root:@127.0.0.1:4000/test')
Base = declarative_base()
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)


class Player(Base):
    __tablename__ = "player"

    id = Column(String(36), primary_key=True)
    coins = Column(Integer)
    goods = Column(Integer)

    def __repr__(self):
        return f'Player(id={self.id!r}, coins={self.coins!r}, goods={self.goods!r})'


def random_player(amount: int) -> List[Player]:
    players = []
    for _ in range(amount):
        players.append(Player(id=uuid.uuid4(), coins=10000, goods=10000))

    return players


def simple_example() -> None:
    with Session() as session:
        # create a player, who has a coin and a goods.
        session.add(Player(id="test", coins=1, goods=1))

        # get this player, and print it.
        get_test_stmt = select(Player).where(Player.id == "test")
        for player in session.scalars(get_test_stmt):
            print(player)

        # create players with bulk inserts.
        # insert 1919 players totally, with 114 players per batch.
        # each player has a random UUID
        player_list = random_player(1919)
        for idx in range(0, len(player_list), 114):
            session.bulk_save_objects(player_list[idx:idx + 114])

        # print the number of players
        count = session.query(func.count(Player.id)).scalar()
        print(f'number of players: {count}')

        # print 3 players.
        three_players = session.query(Player).limit(3).all()
        for player in three_players:
            print(player)

        session.commit()


def trade_check(session: Session, sell_id: str, buy_id: str, amount: int, price: int) -> bool:
    # sell player goods check
    sell_player = session.query(Player.goods).filter(Player.id == sell_id).with_for_update().one()
    if sell_player.goods < amount:
        print(f'sell player {sell_id} goods not enough')
        return False

    # buy player coins check
    buy_player = session.query(Player.coins).filter(Player.id == buy_id).with_for_update().one()
    if buy_player.coins < price:
        print(f'buy player {buy_id} coins not enough')
        return False


def trade(sell_id: str, buy_id: str, amount: int, price: int) -> None:
    with Session() as session:
        if trade_check(session, sell_id, buy_id, amount, price) is False:
            return

        # deduct the goods of seller, and raise his/her the coins
        session.query(Player).filter(Player.id == sell_id). \
            update({'goods': Player.goods - amount, 'coins': Player.coins + price})
        # deduct the coins of buyer, and raise his/her the goods
        session.query(Player).filter(Player.id == buy_id). \
            update({'goods': Player.goods + amount, 'coins': Player.coins - price})

        session.commit()
        print("trade success")


def trade_example() -> None:
    with Session() as session:
        # create two players
        # player 1: id is "1", has only 100 coins.
        # player 2: id is "2", has 114514 coins, and 20 goods.
        session.add(Player(id="1", coins=100, goods=0))
        session.add(Player(id="2", coins=114514, goods=20))
        session.commit()

    # player 1 wants to buy 10 goods from player 2.
    # it will cost 500 coins, but player 1 cannot afford it.
    # so this trade will fail, and nobody will lose their coins or goods
    trade(sell_id="2", buy_id="1", amount=10, price=500)

    # then player 1 has to reduce the incoming quantity to 2.
    # this trade will be successful
    trade(sell_id="2", buy_id="1", amount=2, price=100)

    with Session() as session:
        traders = session.query(Player).filter(Player.id.in_(("1", "2"))).all()
        for player in traders:
            print(player)
        session.commit()


simple_example()
trade_example()

Compared with using drivers directly, SQLAlchemy provides an abstraction for the specific details of different databases when you create a database connection. In addition, SQLAlchemy encapsulates some operations such as session management and CRUD of basic objects, which greatly simplifies the code.

The Player class is a mapping of a table to attributes in the application. Each attribute of Player corresponds to a field in the player table. To provide SQLAlchemy with more information, the attribute is defined as id = Column(String(36), primary_key=True) to indicate the field type and its additional attributes. For example, id = Column(String(36), primary_key=True) indicates that the id attribute is String type, the corresponding field in database is VARCHAR type, the length is 36, and it is a primary key.

For more information about how to use SQLAlchemy, refer to SQLAlchemy documentation.

Step 3. Run the code

The following content introduces how to run the code step by step.

Step 3.1 Initialize table

Before running the code, you need to initialize the table manually. If you are using a local TiDB cluster, you can run the following command:

mysql --host 127.0.0.1 --port 4000 -u root < player_init.sql
mycli --host 127.0.0.1 --port 4000 -u root --no-warn < player_init.sql

If you are not using a local cluster, or have not installed a MySQL client, connect to your cluster using your preferred method (such as Navicat, DBeaver, or other GUI tools) and run the SQL statements in the player_init.sql file.

Step 3.2 Modify parameters for TiDB Cloud

If you are using a TiDB Serverless cluster, you need to provide your CA root path and replace <ca_path> in the following examples with your CA path. To get the CA root path on your system, refer to Where is the CA root path on my system?.

If you are using a TiDB Serverless cluster, modify the parameters of the create_engine function in sqlalchemy_example.py:

engine = create_engine('mysql://root:@127.0.0.1:4000/test')

Suppose that the password you set is 123456, and the connection parameters you get from the cluster details page are the following:

  • Endpoint: xxx.tidbcloud.com
  • Port: 4000
  • User: 2aEp24QWEDLqRFs.root

In this case, you can modify the create_engine as follows:

engine = create_engine('mysql://2aEp24QWEDLqRFs.root:123456@xxx.tidbcloud.com:4000/test', connect_args={
    "ssl_mode": "VERIFY_IDENTITY",
    "ssl": {
        "ca": "<ca_path>"
    }
})

Step 3.3 Run the code

Before running the code, use the following command to install dependencies:

pip3 install -r requirement.txt

If you need to run the script multiple times, follow the Table initialization section to initialize the table again before each run.

python3 sqlalchemy_example.py

Step 4. Expected output

SQLAlchemy Expected Output