Скачать или смотреть How to Create a Composite Primary Key in SQLAlchemy Linked to Another Composite Key

Learn how to effectively manage composite primary keys in SQLAlchemy, especially when one is derived from another composite key.
---
This video is based on the question https://stackoverflow.com/q/64246193/ asked by the user 'Pierre' ( https://stackoverflow.com/u/1503683/ ) and on the answer https://stackoverflow.com/a/64248451/ provided by the user 'Pierre' ( https://stackoverflow.com/u/1503683/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: SQLAlchemy: create a composite primary key made of another composite primary key

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding Composite Primary Keys in SQLAlchemy

When working with relational databases in Python, SQLAlchemy often comes in as a go-to solution for managing your data models. One common challenge is defining a composite primary key made up of another composite primary key. This guide dives into how you can achieve this when dealing with nested relationships.

The Problem: Nested Composite Keys

Imagine a scenario where we have three models: Person, Pet, and Toy. The relationships between them are as follows:

A Person can own multiple Pets.

Each Pet can have multiple Toys.

The structures of these tables would look something like this:

Person table:

id: Unique identifier for each person.

name: The name of the person.

Pet table (Composite Primary Key):

age: The age of the pet.

person_id: A foreign key referencing the Person table.

Toy table (Another Composite Primary Key):

Needs to reference the composite primary key from Pet.

This leads us to the dilemma: how do we structure the Toy table in a way that leverages the composite primary key from the Pet model?

The Solution: Defining Composite Primary Keys

To achieve this in SQLAlchemy, follow these steps:

1. Define the Person Model

Start off by defining your Person class. This is straightforward as it only has a single primary key:

[[See Video to Reveal this Text or Code Snippet]]

2. Define the Pet Model

For the Pet table, which requires a composite primary key, we'll declare it as follows:

[[See Video to Reveal this Text or Code Snippet]]

In the above code:

We define a composite key consisting of age and person_id.

3. Define the Toy Model

Here comes the tricky part with the Toy table. Since we need to reference the composite key of the Pet, we must define the additional fields to make this possible:

[[See Video to Reveal this Text or Code Snippet]]

Key Details in the Toy Model:

Composite Primary Key: The combination of color, pet_age, and person_id creates a unique identifier for each toy.

Foreign Key Constraint: This ties the Toy table to the Pet table, enabling us to use the pet_age (from the Pet composite primary key) and person_id together.

Conclusion

By understanding the nested relationships and properly defining your SQLAlchemy models, you can effectively manage composite primary keys that depend on other composite keys. This structured approach allows you to create robust database schemas that maintain referential integrity while supporting complex relationships within your data.

Make sure to follow these steps correctly to establish your models without running into issues, and feel free to reach out with any further questions!