Add support for Index Length for MySQL-backed Models

[umbrae@…]

On occasion, it is useful to be able to specific a key length different than the field length for a field - this is particularly true with MySQL, whose index sizes are limited to 767 bytes in InnoDB.

For example, I have a simple field like the following:

url = models.CharField(max_length=767, db_index=True)

Where all fields are UTF-8 characters, and hence 3 bytes each. This will issue a warning when I try to syncdb (and will full-on fail in south, as south bombs out on warnings):

_mysql_exceptions.Warning: Specified key was too long; max key length is 767 bytes

What I'd like to be able to do is the following:

url = models.CharField(max_length=767, db_index=True, index_length=255)

Which will then change the CREATE INDEX sql to look like:

CREATE INDEX "some_table_a4b49ab" ON "some_table" ("url"(255));

This should no longer issue a warning in MySQL.

[umbrae@…]

(Addendum: I know that, in general, such long indexes are discouraged - they are sometimes necessary however.)

[Aymeric Augustin]

The idea makes sense, and wasn't reported before. I'm not sure if we want to add this in Django, though.

[Malcolm Tredinnick]

Adding database specific attributes to Field classes isn't an option. We already allow initial SQL as well as emitting a syncdb signal that can be used for just this purpose: custom creation SQL insertion. That is where you would put code to create the appropriate index.