Hi.
At the begining please forgive my engilsh - i'm not a native speaker.
I wrote here and in other places my thoughts about db migrations few times,
and probably Tim remembers me so well.
My opinion was not changed, but I realized that I cannot leave Django
ecosystem for a long time.
In that case I'd like to talk about migrations, their advantages and
disadvantages, and about possible solutions.
Advantages:
- fast (automatic) creation
- database independent (model-centric)
- a standard for django itself and reusable apps
- possibilty to create migrations manually
Disadvantages:
- dependent on application layer (a python code - field, model classes,
etc)
- allowing python code within migrations
- separate files for changesets makes ugly conflicts when merging
branches
- squashing required
I'd like to focus on disadvantages, because they're a casue of using
alternative solutions by me (Liquibase in that case).
*Application layer dependency*
This is something whch causes fails of whole migration system.
References to the application layer are included within migration files,
because of saving a "model state".
Any significant code change will broke migrations. We must avoid such
situations by squashing migrations at "the right time".
In my opinion migrations should be application independent. Unfortunatelly
whole system is based on models written in Python, which may includue
custom solutions (i.e. model fields).
I understand that it is hard to cut-out this feature, but I believe that
there exist some solution, which drops app layer dependency ands allow
using custom fields.
*Python code in migrations*
This is a generally bad idea. Any python code is strictly related to the
time. When code changes, a "pythonic" migration may fail.
And you will never know about failure until you setup CI properly.
There are very rare cases, when something from app layer must be called
between releases.
In that cases I'm using management commands, but Liquibase allows me to
execute any binary.
It is not so straightforward, so as a developer you feel that you're doing
something strange/non-standard, and you should be careful.
The general problem is that some migrations stops working properly in the
time.
This should never happen. Well... I'm used to this and I adhere to this
principle.
I'm using Liquibase for years and my migrations broke few times mostly on
changesets based on binaries execution (a python code through manage
commands).
*Separate files for changesets*
I like Liquibase because of possibility of using files containing more
changesets.
When I need to split migration files, I can do that and use "include"
directive.
There are advantages: easy conflict resolution, just one file per database
and per current major release, and full changes history in older files.
Django produces spearate migraiton files. This causes conflicts and
reordering migrations by declarations ("depends").
And because squashing is almost required for long project, you're loosing a
changesets history.
*Squashing required*
For long-live projects this is a required operation. Squashing deletes
changes history and removes obsolete python code from migrations.
Squashing is a just a workaround tool for migrations design issues.
*What I need?*
Well, I'd like to simplify my work. That's obvious. Maintaining a Liquibase
changes outside Django is harder and requires more time, especially when
I'm adding new apps or upgrading existing ones.
But I don't want to switch back to the Django migrations, because of the
design issues described above (mostly beacuse of the app layer dependency
and squashing requirement).
I started prototyping a tool which translates Django migrations into pure
SQL, which can be embedded in a Liquibase changesets.
But Python migrations can't be translated to SQL, of course. And the worst
thing is that Django provides Python migrations even for contrib apps.
In that case I realised, that building such tool without changing a concept
of the Django migrations (by you - Django Developers), is a little
unreasonable, until Python code execution is accepted and used internally
(contenttypes, 0002).
*What I would like to achieve by this post?*
A discussion about removing app layer dependency and removing or limiting
RunPython usage, mostly.
This should eliminate requirement of squashing and increase migrations
stability.
Separate files vs big file is not important now. This is just unhandy, but
does not produce failures.
Kind Regards,
Marcin
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