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Postgres Foreign Key Constraints

Nice article on django migrations!

A missing major item is adding foreign key constraints. Adding them on a large table can cause havoc on both the base and foreign table. It'd be great to add a section about this to your post.

Why " Adding a default to an existing column also doesn’t require an expensive table rewrite "

Awesome django migrations article !

But I have a quesiton . In "Add the default to the database schema" section , Why you said " Adding a default to an existing column also doesn’t require an expensive table rewrite. "

When Postgres exec ALTER TABLE users ALTER COLUMN blocked SET DEFAULT false ， will It not traverse the whole table ?

Post-deployment migrations support

Hello @Pankrat, I wanted to reach out to let you know that you might be interested in https://github.com/charettes/django-syzygy which implements the last missing point on your Django wishlish from your 2015 posts.

I've been actively contributing to Django's migration framework over the years and plan on trying to eventually get the concept of pre and post deployment migrations into core once I received appropriate feedback on the idea.

I know it's been a while but I figured you might be interested in learning about the development on the topic.

Django Migration Idempotent Clarification

Hi,

Thanks for sharing this blog post. I just wanted to point out one issue. In the post, you state

As Django migrations are idempotent

This statement is misleading and not accurate. It's fair to say that under certain happy path conditions Django's migration commands are idempotent, but the migrations themselves are not. For example, you cannot re-run a specific migration that creates a table because the compiled SQL code uses CREATE TABLE. If the migration itself were idempotent, it would use CREATE TABLE IF NOT EXISTS and so on.

Moreover, Django migrations are stateful (state is tracked in thedjango_migrations table) and IMO statelessness is an important property of an idempotent system.

Thanks,

Michael

Good job!

Keep it up!

non atomic migrations

https://docs.djangoproject.com/en/1.11/howto/writing-migrations/#non-atomic-migrations

On databases that support DDL transactions (SQLite and PostgreSQL), migrations will run inside a transaction by default. For use cases such as performing data migrations on large tables, you may want to prevent a migration from running in a transaction by setting the atomic attribute to False: