Error display when makemigrations' field serializing fails with ValueError

[torstenrudolf]

In django1.8.4 the error message displayed, when the field serialization fails (e.g. because there is a lambda function used for one argument) is not very helpful.

It would be nice to know on which field the error happened.

a simple patch could be inside django.db.migrations.writer to wrap line 377 (seems to be 422 in current master ​https://github.com/django/django/blob/e34226fc37dfa9eba89d913fd7ab8e95663b0d64/django/db/migrations/writer.py#L422)

elif isinstance(value, models.Field):
    attr_name, path, args, kwargs = value.deconstruct()
    return cls.serialize_deconstructed(path, args, kwargs)

into a try-except block like this:

elif isinstance(value, models.Field):
    attr_name, path, args, kwargs = value.deconstruct()
    try:
        return cls.serialize_deconstructed(path, args, kwargs)
    except ValueError as e:
        e.args = ('During serialization of the field {} the following blew up: {}'.format(value, e.args[0]), ) + e.args[1:]
        raise e, None, sys.exc_info()[2]

end of the traceback before the change:

...
.../local/lib/python2.7/site-packages/django/db/migrations/writer.py", line 415, in serialize
    raise ValueError("Cannot serialize function: lambda")
ValueError: Cannot serialize function: lambda

after the change:

...
.../local/lib/python2.7/site-packages/django/db/migrations/writer.py", line 419, in serialize
    raise ValueError("Cannot serialize function: lambda")
ValueError: During serialization of the field <app_label>.<model_name>.<field_name> the following blew up: Cannot serialize function: lambda

[Simon Charette]

Makes sense to me, patch is welcome!

[Adam]

working on ​https://github.com/awwester/django/tree/ticket_25370

[Mariusz Felisiak]

I think we can deassign after 5 years.

[Tameesh Biswas]

May I pick this?

[Mariusz Felisiak]

Of course, there is no need to ask.

[Ram Parameswaran]

This module (django/db/migrations/serializer.py) was overhauled and merged in 2016 (see ​https://github.com/django/django/pull/6059).
Do you think a patch is still required?

[Mariusz Felisiak]

Yes it's still valid, the same exception is raised in django/db/migrations/serializer.py.

[Craig Smith]

I didn't see any patch for this so I have added one with this ​PR . Tameesh, may I reassign this?

[Mariusz Felisiak]

​PR

[Mariusz Felisiak]

Sample project.

[Mariusz Felisiak]

It looks that it's not feasible to get <app label>.<model name>.<field name> or even <app label>.<model name> in a reliable way because Django serializes instances of fields from django.db.models not a model attributes. Each approach doesn't work in some cases, e.g. constructing messages in the FunctionTypeSerializer will not work for lambdas defined in the module

Error during serializing test_one.models.<lambda>:  ...

or for lambdas imported from other modules:

ValueError: Error during serializing test_one.utils.<lambda>: ...

instead of ValueError: Error during serializing test_one.models.Model1.field1.