#28949: Multibyte table name or column name causes miscalculation of the length
of
index name.
-------------------------------------+-------------------------------------
Reporter: Pak | Owner: nobody
Youngrok |
Type: Bug | Status: new
Component: | Version: 2.0
Migrations | Keywords: migration multibyte
Severity: Normal | index
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
-------------------------------------+-------------------------------------
Django migration automatically creates index with name consists of table
name, column names, hash, and suffix. When the length of generated index
name is greater than `self.connection.ops.max_name_length()`, it shortens
the name. However, it calculate length as python string type, so it's
length doesn't match with the length of databases. The length should be
calculated after encoded with the database encoding. Because of this
issue, migration fails with these conditions below:
* long multibyte model names
* two multibyte model related with foreign key
* the foreign key field is CharField(or it's child class)
With these conditions, django migration tries to create two index(one for
normal index, one for `like` index), and the name of those are same except
suffix(the latter has suffix `_like`), and the lengths of both index names
as string are less than max name length but the length of both index names
as bytes are greater than max name length, so name conflict is raised.
long multibyte table name and foreign key name.
Here is the code:
https://github.com/django/django/blob/4420761ea9457d386b2000cf9df5b2f6f88f8f91/django/db/backends/base/schema.py#L873
{{{#!python
index_name = '%s_%s_%s' % (table_name, '_'.join(column_names),
hash_suffix_part)
if len(index_name) <= max_length:
return index_name
}}}
[https://docs.djangoproject.com/en/2.0/ref/databases/#encoding Django
assumes that all databases use UTF-8 encoding], so the code should be
fixed like this:
{{{#!python
index_name = '%s_%s_%s' % (table_name, '_'.join(column_names),
hash_suffix_part)
if len(index_name.encode('utf8')) <= max_length:
return index_name
}}}
The code that shorten the name should be also fixed. Getting a third of
each part and re-joining is not good strategy in multibyte world, it can
also cause miscalculation. I think getting very small amount of table and
column names like 2 or 3 characters and joining them with original hash
can be a safe solution.
--
Ticket URL: <https://code.djangoproject.com/ticket/28949>
Django <https://code.djangoproject.com/>
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