FileField needlessly serializes upload_to

[Roman Odaisky]

django.db.models.fields.FileField explicitly includes upload_to in the serialization process. This takes no part in the actual migration process but causes errors if the value of the field is a lambda function. Instead, the deconstruct() functions for FileField and other fields should only serialize arguments relevant to subsequent ALTER commands issued to the DBMS.

Once fixed, #22351 can be reverted.

[Roman Odaisky]

Alternatively, a better way might be to serialize lambdas to something that raises NotImplementedError. Also, given that upload_to and other fields might matter in case migrations contain RunPython blocks, a warning could be appropriate if both unserializable non-essential fields and RunPython are encountered.

Anything would be preferable to the current situation.

[Tim Graham]

The documentation describes why options like upload_to are serialized:

Django will make migrations for any change to your models or fields - even options that don’t affect the database - as the only way it can reconstruct a field correctly is to have all the changes in the history, and you might need those options in some data migrations later on (for example, if you’ve set custom validators).

I'm not convinced that allowing lambdas for some options where it may not be needed (like upload_to), but not in other places where it is needed (like default) would be a good situation. Could you please raise the issue on the DevelopersMailingList if you think it merits further discussion? Thank-you.