sqlsequencereset should inform that no sequences found.

[Keryn Knight]

This just came up on IRC, because someone was expecting sqlsequencereset to provide resets for the auto-increment values for an SQLite table.

Running python manage.py sqlsequencereset <myapp> provides no output if there are no results returned by connection.ops.sequence_reset_sql (see ​here)
So the user has no idea if it failed, succeeded, or they got the invocation wrong (assuming they're not familiar enough with Django to know that invoking it wrong will raise a CommandError).

I'd suggest it should avoid ambiguity, so if len(statements) == 0 it should raise CommandError and say there's nothing to do. Bonus points if it also sniffs the connection backend to know if there is anything it could do, and if there's not, report that sqlsequencereset isn't necessary/available for that backend.

[Mariusz Felisiak]

Thanks for the report. I don't think that sqlsequencereset should raise CommandError, but I agree that we can add an extra output if we don't find any sequences, e.g.

if not statements and self.verbosity >= 1:
    self.stdout.write('No sequences found.')

[Simon Charette]

One problem I foresee with outputting a message when no sequences are found is that it could break scripts piping the output into a SQL shell or simply expecting STDOUT to be valid SQL. I guess we could output an SQL comment of the form -- No sequences found or use STDERR instead?

Also I don't see why sqlsequencereset is special with regards to other sql* commands that also have the same behavior; it should either not be changed or changed for all sql* commands to be coherent.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 5d03f2b:

Fixed #30595 -- Added error message when no objects found to sql* management commands.