In SQLAlchemy, creating a model involves defining a Python class that corresponds to a table in the database. Each instance of this class represents a row in the table. Let's walk through a simple example where we create a Comment model for storing user comments.

from sqlalchemy import Column, Integer, String, Text, DateTime
from sqlalchemy.ext.declarative import declarative_base
from datetime import datetime

Base = declarative_base()

class Comment(Base):
    __tablename__ = "comments"  # the name of the table in the database

    id = Column(Integer, primary_key=True)  # primary key column, uniquely identifies each comment
    content = Column(Text, nullable=False)  # content of the comment, cannot be null
    created_at = Column(DateTime, default=datetime.utcnow)  # timestamp when the comment is created, defaults to current time

# Create all tables defined by models
Base.metadata.create_all(engine)

This defines a Python class called Comment, which represents a table in the database. The class inherits from Base, which enables it to be mapped to a table in the database.

__tablename__ = "comments"

The Comment class, inheriting from Base, defines a table in the database. This allows it to be mapped to a database table using Object-Relational Mapping (ORM).

id = Column(Integer, primary_key=True)

The id field serves as the primary key, ensuring that each comment has a unique identifier. SQLAlchemy automatically increments the value for each new record.

content = Column(Text, nullable=False)

The content field contains the text of a comment. It is a required field, meaning that a comment can't be added to a database if this field is null.

created_at = Column(DateTime, default=datetime.utcnow)

The created_at field automatically stores the timestamp for when the comment was created. If no value is provided, it is set to the current time using datetime.utcnow, ensuring the timestamp is recorded in UTC.

Base.metadata.create_all(engine)

This line creates all the tables in the database that were defined in the models. It uses the information from the metadata and creates the necessary tables through the connection provided by the engine. This is useful for automatically creating tables if they don't already exist.