#24591: Copy ModelState.fields and ModelState.managers instead of clone
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Reporter: knbk | Owner: knbk
Type: Cleanup/optimization | Status: new
Component: Migrations | Version: master
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 1
Easy pickings: 0 | UI/UX: 0
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Fields in a`ModelState` are guaranteed to not be bound to a model, and are
deconstructed and reconstructed in `ModelState.from_model()`. They are
also properly deconstructed and reconstructed when rendering a model.
Furthermore, the fields themselves must not change during the lifetime of
a `ModelState` object, though you may freely add, remove and replace
fields (1). They should be considered immutable within model states.
I think the current implementation of `ModelState.clone()` is too
conservative in this regard. Both the input (in `ModelState.from_model()`)
and the output (in `ModelState.render()`) do a full de- and reconstruct.
The only change in between is the addition and removal of fields from the
list. `ModelState.clone()` can do a simple `list(self.fields)` and still
maintain the correct behaviour. The same holds for `ModelState.managers`.
This change saves about 75% in rendering time, and 66% in memory usage in
my benchmarks.
If you want to safeguard against accidental changes that can affect other
model states, you can use `copy.deepcopy()`, though at a performance cost.
Other options are to somehow shield against accidental changes by
generating an immutable field class, or by saving the deconstructed field
and constructing the fields on demand.
As this is ''the'' most performance-critical part of migrations -- save
the actual database manipulation -- I suggest we take a careful look into
this. The current implementation with `deconstruct()` is far from the
ideal solution.
(1)
https://github.com/django/django/blob/master/django/db/migrations/state.py#L294
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Ticket URL: <https://code.djangoproject.com/ticket/24591>
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