FastAPI SQLAlchemy Relationships Update Model Data Relations

Learning Objective:
By the end of this lesson, you will be able to implement model data relationships in a SQLAlchemy model.

Updating the TeaModel to Relate to Comments

To link the TeaModel and CommentModel, we need to establish a relationship between these models. This involves updating the TeaModel to include a comments property that represents the relationship.

This property uses SQLAlchemy’s relationship method to manage how the two tables (teas and comments) are connected in the database.

Update TeaModel

Here’s the updated code for TeaModel:

# models/tea.py
from sqlalchemy import Column, Integer, String, Boolean
from sqlalchemy.orm import relationship  # Import the relationship function from SQLAlchemy ORM
from .base import BaseModel  # Import the base model for SQLAlchemy
from .comment import CommentModel  # Import the CommentModel class for establishing relationships

# Update Base to BaseModel
class TeaModel(BaseModel):

    __tablename__ = "teas"

    id = Column(Integer, primary_key=True, index=True)

    # Specific columns for our Tea Table.
    name = Column(String, unique=True)
    in_stock = Column(Boolean)
    rating = Column(Integer)

    # Define a relationship with the CommentModel table
    comments = relationship("CommentModel", back_populates="tea")

Let’s break down the important updates:

1. Importing CommentModel

   • At the top of the file, we imported the CommentModel.
   • This allows us to reference it in the relationship method.

2. Adding the comments Property

   • The comments property is defined using SQLAlchemy’s relationship method.
   • This property links TeaModel to CommentModel, telling SQLAlchemy to manage their connection.

   comments = relationship("CommentModel", back_populates="tea")
   

Why Use back_populates?

The back_populates attribute ensures that the relationship works in both directions:

• In TeaModel: The comments property retrieves all comments related to a tea.
• In CommentModel: The tea property links back to the tea that a comment belongs to.

By defining back_populates in both models, we create a synchronized two-way connection.