Скачать или смотреть Understanding SQLAlchemy Cascading Deletes: When Do You Need to Use Relationships?

Learn how SQLAlchemy interacts with PostgreSQL and understand the importance of relationships and cascade deletes in SQLAlchemy.
---
This video is based on the question https://stackoverflow.com/q/63987133/ asked by the user 'satinder singh' ( https://stackoverflow.com/u/8486505/ ) and on the answer https://stackoverflow.com/a/64678254/ provided by the user 'satinder singh' ( https://stackoverflow.com/u/8486505/ ) 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: data gets deleted without any cascade delete options added to sql alchemy

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 SQLAlchemy Cascading Deletes: When Do You Need to Use Relationships?

When working with databases, maintaining the integrity of your data is crucial, especially when it comes to relationships between tables. One common problem developers encounter is ensuring that deleting a row from one table automatically removes related rows from another. This is known as a cascading delete. Today, we explore a particular scenario with SQLAlchemy and PostgreSQL, clarifying how cascading deletes function and whether you need to explicitly define relationships.

The Scenario

Imagine you have a PostgreSQL database with three interconnected tables:

one: Contains general user information

two: Relates to table 'one' with a foreign key

three: Relates to table 'two', continuing the chain of relationships

The structure could look like this:

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

With this setup, your goal is to ensure that when a record in table "one" is deleted, all corresponding records in tables "two" and "three" should also be deleted automatically. You already have the ON DELETE CASCADE constraint configured in your Postgres database, which works correctly through direct SQL queries in the CLI.

The Confusion with SQLAlchemy

The tricky part arises when you try to implement the same behavior using SQLAlchemy. According to the SQLAlchemy documentation, you should define relationships and set cascade options like this:

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

However, it seems this is not necessary since the cascading delete functionality is still working without these specifications when you issue delete commands via SQLAlchemy.

The Explanation Behind It: SQLAlchemy and Database Constraints

To clarify:

SQLAlchemy is essentially a wrapper over your database. It translates your Python code (ORM) into SQL commands that the underlying database understands.

If your database already has cascade delete constraints defined, SQLAlchemy will follow those rules. So when you delete a record from one table, the database engine takes care of the deleted entries in related tables based on the constraints you've set.

Why Does it Work Without Explicit Relationships?

Database Control: The database is the one enforcing the constraints. SQLAlchemy respects these constraints—it does not override the behavior dictated by the actual database schema.

Focus on Simplicity: If your requirements can be fulfilled with the existing constraints, it doesn't make sense to add additional layers of complexity to your code with unnecessary relationships.

Use Cases: Explicitly defining relationships becomes more critical when you need to use the ORM’s features, such as ordering queries or eager loading related records.

Conclusion

In summary, while SQLAlchemy offers powerful tools for managing relationships and cascades, it is essential to understand that it does not control database behavior but respects its constraints. In scenarios like the one described above, relying on your database's cascade delete functionalities is often sufficient.

If your use case has more complex inter-table dependencies or specific ORM features you want to leverage, then it would be beneficial to explore defining relationships properly. Otherwise, remember that good database design with appropriate foreign key constraints can handle a lot of the heavy lifting for you!