#31051: Serialization dependency sorting disallows circular references
unneccesarily
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Reporter: Matthijs Kooijman | Owner: nobody
Type: Bug | Status: new
Component: Core (Serialization) | Version: master
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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The `core.serialization.sort_dependencies()` function takes a list of apps
and/or models, and resolves this into a sorted flat list of models, ready
to be serialized in that order. This function is intended to make natural
foreign keys work, by serializing models referenced by a natural key
before the referencing models. When deserializing, this guarantees that
natural key references can be resolved, because there are no "forward
references". Furthermore, when a circular reference using natural keys is
present, this function raises an exception (e.g. "Can't resolve
dependencies for some_app.SomeModel in serialized app list") and prevents
serialization from completing, since there is no way to guarantee a model
ordering that will have no forward references.
Note that this ordering is *only* needed when natural keys are involved,
since data is intended to be loaded in a transaction without constraint
checks, so numerical foreign keys can be added in the wrong order, as long
as all referenced data is present at the end of the transaction. This does
not work with natural keys, since those are resolved by Python code that
needs the referenced objects present in the database to resolve them.
However, this sorting is not actually strictly necessary in all cases
where it is applied. When circular references are involved, this then
actually prevents serialization for no good reason. In particular, this is
the case:
- When running `dumpdata` without natural keys enabled (which is the
default). Even though natural keys might be defined in the models (which
causes the sorting and exception), no natural keys will be present in the
dumped data, so no ordering is needed.
- When dumping data intended for loading with `loaddata` (which I think
is the primary usecase for `dumpdata`?). `loaddata` will (since 17 months
ago in v2.2, see #26291) automatically handle forward references by
deferring setting fields that reference natural keys that are not added
yet. In this case, sorting is still useful, to prevent forward references
where possible, but when there are circular references, it is acceptable
to ignore some dependencies rather than preventing serialization from
happening alltogether.
- When serializing data for tests for `serialized_rollback=True` (in
`django.db.backends.base.creation.create_test_db`). This is a
serialization that does not use natural keys, so no ordering is needed at
all. Note that this serialization happens always (unlike deserialization
only happens with `serialized_rollback=True`), so AFAIU this effectively
prevents *any* tests from working on a database with circular references
with natural keys defined.
The fix for these issues seems to be rather simple:
- For dumpdata without `use_natural_foreign_keys`, skip the ordering and
just serialize all models in arbitrary order. AFAICS
`use_natural_primary_keys` is not relevant here, since that only controls
omitting the numerical primary key.
- For dumpdata *with* `use_natural_foreign_keys`, do the ordering but do
not bail out when there are circular references (instead just ignore some
dependencies and produce a best-effort ordering).
- For test database serialization, also skip the ordering and serialize
in arbitrary order.
Note that this would remove two of the three calls to
`sort_dependencies()` and allow loops in the last remaining instance. This
means that `sort_dependencies` could be modified to allow loops
unconditionally, or we could add an argument and default to disallowing
loops in case any code outside of django is using this function?
Note that #26552 is a related, but different issue, concerning the
*deserialization* of data in testcases.
I've been working on fixing this and that related issue today and have a
basic version working, with testcases (which proved to be quite a
challenge, since testing the test subsystem is a bit tricky...). I'll do
some additional testing and cleanup and submit a PR soon.
Also note that the circular-reference exception was already disabled for
self-referencing models in #16317. The fix for that issue simply ignores
self-referencing models for sorting, without taking any additional
measures to sort instances to prevent problems in deserialization (this
code was added when the deferred deserialization did not exist yet), so I
wonder how much value this exception still has.
--
Ticket URL: <https://code.djangoproject.com/ticket/31051>
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