Re: Why does sqlmigrate need a connection to an existing database?

To any future readers, the real answer is that connections are deeply baked into how Django migrations operate. Things as seemingly simple as `can_migrate_model` require a connection. The other problems pointed out by Jason are easily patched, but the schema_editor (and all of the migration operations in migrations/operations which rely on the schema_editor) totally break without a connection.

I think it could be done, but it isn't an easy patch.

On Saturday, 11 December 2021 at 07:59:32 UTC-8 Jason wrote:

https://github.com/django/django/blob/main/django/db/migrations/loader.py#L20-L40

the comment in the migration loader class probably explains your question.  also, look at `collect_sql` at the bottom, and how its used at the end of sqlmigrate. specifically, it uses schema editor (https://docs.djangoproject.com/en/3.2/ref/schema-editor/#module-django.db.backends.base.schema) as a context manager to have the connection turn migration nodes to SQL.

On Friday, December 10, 2021 at 12:06:21 PM UTC-5 shmuel....@partnerize.com wrote:

Build graph has a specific separate path for `if self.connection is None:`, so that shouldn't be an issue?
As far as atomicity, what if, in a case where no connection is found, it could just prompt: "No database connection found. Do you want your migration to be atomic? (y/N)"

On Thursday, 9 December 2021 at 05:01:31 UTC-8 Jason wrote:

It uses db connection in two places:

building the migration graph in order to load previous applied migrations and transaction begin/end wrapping

This makes sense, because you can have N unapplied migrations between the db and your migrations history package.  

On Tuesday, December 7, 2021 at 7:46:42 PM UTC-5 shmuel....@partnerize.com wrote:

Wondering why sqlmigrate needs a connection to an existing database.

I understand that for certain commands, it needs the database to generate the migration. But for most (basic) commands, no connection is really needed.

For example, I created a test project and generated migration files. Without ever running the `migrate` command, I was able to use `sqlmigrate` perfectly well on all migrations, save the last, which drops a `unique_together`.

I'm sure there are other commands which actually need a database connection (though I haven't found them yet). Seems to me sqlmigrate could be rewritten along the lines of:

• Try to generate sql from migration file
  
• If it hits a command it needs the database, attempt to connect

Or else:

• If no connection, check if migration file needs connection to database
  

Is there something I'm unaware of?

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