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