FastAPI SQLAlchemy Relationships Defining a Comments Model

Learning Objective:
By the end of this lesson, learners will be able to define a second, related SQLAlchemy model class for storing and managing comments data.

Creating the comment model

In this lesson, we will create a CommentModel class to represent comments and their relationship to a tea. This will involve defining attributes for the comment and establishing a connection to the tea it references.

First, create a new file named models/comment.py.

touch models/comment.py

Inside this file, define the CommentModel class as shown below:

# models/comment.py
from sqlalchemy import Column, Integer, String, ForeignKey
from sqlalchemy.orm import relationship
from .base import BaseModel

class CommentModel(BaseModel):

    __tablename__ = "comments"  # The name of the table in the database

    id = Column(Integer, primary_key=True, index=True)  # Unique identifier for the comment
    content = Column(String, nullable=False)  # The text content of the comment

    # ForeignKey establishes a connection to the teas table
    tea_id = Column(Integer, ForeignKey('teas.id'), nullable=False)
    tea = relationship("TeaModel", back_populates="comments")  # Defines the relationship to the TeaModel

The comment model specifies three attributes:

• A unique id for the comment (integer)
• The comment content (string)
• The tea_id that the comment pertains to (integer)

What is a foreign key?

A foreign key is a special database constraint used to connect two tables. It ensures that a value in one table (like tea_id in the comments table) corresponds to a valid value in another table (like id in the teas table).

For example:

• If tea_id in a comment is 5, then there must be a record in the teas table where id is 5.
• This ensures that each comment always belongs to an existing tea.

Tip: The string passed to ForeignKey must match the __tablename__ of the related table and the name of its primary key. For example, ForeignKey('teas.id') links the comments table to the id column in the teas table.

With this model, you now have a structure to manage comments data and link it to teas in your application.