Re: A lot of Problems with Migrating (conceptual)

Excellent - thanks much.

On Aug 18, 2017, at 4:02 PM, knbk <marten.knbk@gmail.com> wrote:

There are various reasons why you should never execute queries at startup. One you've seen: it makes it impossible to migrate a new database. Other reasons include that the choices are cached and are only renewed when the server restarts, and that when running tests, the query is executed before the test database is set up and will run on the main database. 

The solution in this case is to use a ModelChoiceField[1], i.e.:

    category = forms.ModelChoiceField(queryset=Category.objects.all(), required=False)

This allows to pass in a queryset which is evaluated whenever the field is used to make sure the values are up-to-date. Since querysets are lazy, this won't execute any queries on startup. 

[1] https://docs.djangoproject.com/en/1.11/ref/forms/fields/#modelchoicefield

On Saturday, August 19, 2017 at 12:14:48 AM UTC+2, subaru_87 wrote:

Here's a "problem" with migrations that I've had. It only occurs on a first time installation (like a first-time upload to Heroku, for example).

In forms.py, I have a field dependent upon a column in another table (Category):

    category = forms.TypedChoiceField(required=False, choices=[(category.pk, category.name) for category in Category.objects.all()])

Running makemigrations or migrate if there's no pre-existing database with a table named "Category" gives an error saying as much. However, I'm expecting migrate to create the tables required. Therefore, it's a catch-22. 

Of course, the "easy" answer is to comment out the above code, add in:

     category = forms.CharField(max_length=12) 

And run the initial migrations. Then, turn around and comment out that same code, uncomment the real code shown above, and run migrations again. It works, but it seems a bit awkward.

Has anyone else had this issue? Or am I doing something wrong?

Thanks much,

Jim Illback

On Aug 18, 2017, at 1:21 AM, James Bennett <ubern...@gmail.com> wrote:

On Thu, Aug 17, 2017 at 1:03 PM, Antonis Christofides <ant...@djangodeployment.com> wrote:

Second, just to make things clear, the word "migration" has two meanings. The original meaning of migration is to switch to another software system (e.g. migrate from MySQL to PostgreSQL, or migrate a repository from subversion to git). In Django, the term "migration" means something else: to update the database to the new schema. In my opinion this terminology is wrong and confusing (apparently it comes from Ruby on Rails, but I'm not certain), and a better one would have been "dbupdate" or something, but since it's migration we'll keep it and you'll have to understand which meaning is meant in each case.

An important thing worth knowing is that Django's migration framework does *not* only track the database schema. It also tracks the state of the Python classes which define the models, including options which do not affect the underlying DB schema, and making such changes to a model will create the need for a migration.

This is absolutely necessary, though, because the migration framework generates a frozen snapshot of the state at each migration, so that you can access the ORM to do things like data updates via the RunPython migration operator. If you were to change, say, the default ordering of a model without also generating a migration to record that change (even though it has no effect on the database schema), you could run into trouble if you later removed a field referenced by the old ordering, since one of the intermediate ORM snapshots would attempt to order queries by a now-nonexistent column.

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