Silent data loss when decreasing the max_length of a CharField

[Gavin Wahl]

I have a CharField that I want to decrease the max_length on. I change the value and generate a migration, then run it on my database.

Desired:

I should get an error from postgres if existing data doesn't fit in the new length:

ERROR: value too long for type character varying(5)

Observed:

Any existing data longer than the new_max length is truncated and lost.

This behavior was introducted in #25002, which unconditionally added a cast to the new type. This changes the default behavior of postgres (which is to never lose data), to implicitly truncate. Django should not lose data by default.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 1378d665:

Fixed #28816 -- Prevented silencing data loss when decreasing CharField.max_length on PostgreSQL.

[Mariusz Felisiak]

This solution doesn't prevent silencing data loss when decreasing CharField.max_length for a base field of ArrayField (see ​comment).

[Simon Charette]

Or decreasing the size of an ArrayField.

[Hasan Ramezani]

Replying to felixxm:

This solution doesn't prevent silencing data loss when decreasing CharField.max_length for a base field of ArrayField (see ​comment).

​PR

[Hasan Ramezani]

Reopen the ticket because of @felixxm comment.

[Hasan Ramezani]

The patch was changed based on @felixxm ​comment on the PR.

[Mariusz Felisiak <felisiak.mariusz@…>]

In ef4beafa:

Refs #28816 -- Prevented silencing data loss when decreasing CharField.max_length for ArrayField.base_field on PostgreSQL.