#35976: Persist original Meta class on Options instances
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Reporter: Tim McCurrach | Owner: (none)
Type: Uncategorized | Status: new
Component: Documentation | Version: 5.1
Severity: Normal | Resolution:
Keywords: | Triage Stage: Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 1 | UI/UX: 0
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Changes (by Tim McCurrach):
* component: Database layer (models, ORM) => Documentation
* resolution: wontfix =>
* status: closed => new
* type: New feature => Uncategorized
Old description:
> When a `model.Model` class is created
> [https://github.com/django/django/blob/ded485464214a3f69b64402b7d82221279f80008/django/db/models/base.py#L112
> the metaclass `pop`s off] any `Meta` attribue defined on the model
> [https://github.com/django/django/blob/ded485464214a3f69b64402b7d82221279f80008/django/db/models/base.py#L122
> before calling] `super.__new__()`. We do
> [https://github.com/django/django/blob/ded485464214a3f69b64402b7d82221279f80008/django/db/models/base.py#L145
> pass it as an argument] to the `Options` class, and
> [https://github.com/django/django/blob/ded485464214a3f69b64402b7d82221279f80008/django/db/models/options.py#L136
> it's stored] on the `Options` instance, however
> [https://github.com/django/django/blob/ded485464214a3f69b64402b7d82221279f80008/django/db/models/options.py#L232
> this is then deleted] whilst the `Options.contribute_to_class` class is
> called.
>
> As far as I can tell, there is no reason why it is important to delete
> the original data stored in the class defined on the original model.
> Admitedly, perhaps it could be stored with a better name to indicate it
> is a non-functional reference to the original `Meta` class.
> `instance._meta.meta` probably doesn't make much sense, but
> `instance._meta.original_meta` might be more appropriate.
>
> === Motivation for feature request ===
>
> The motivation for this came change is born out of a desire to write a
> check that `db_table` is explicitly defined on all my models (even if it
> matches the default value that django would apply). I can write checks
> that require things like `ordering` is defined (or not defined) on a
> model. The problem with `db_table` is that defaults are applied when
> `contribute_to_class` is called. This means we need a reference to the
> underlying original Meta data. This is not currently possible without
> subclassing the metaclass (`ModelBase`) which is a step further than
> would be ideal imo.
>
> === The fix ===
>
> The fix should be very simple. We would just need to remove (or adapt)
> the line of code linked above which deletes the meta-class. (We could
> maybe use a better name than `original_meta`!!)
New description:
When a `model.Model` class is created
[https://github.com/django/django/blob/ded485464214a3f69b64402b7d82221279f80008/django/db/models/base.py#L112
the metaclass `pop`s off] any `Meta` attribue defined on the model
[https://github.com/django/django/blob/ded485464214a3f69b64402b7d82221279f80008/django/db/models/base.py#L122
before calling] `super.__new__()`. We do
[https://github.com/django/django/blob/ded485464214a3f69b64402b7d82221279f80008/django/db/models/base.py#L145
pass it as an argument] to the `Options` class, and
[https://github.com/django/django/blob/ded485464214a3f69b64402b7d82221279f80008/django/db/models/options.py#L136
it's stored] on the `Options` instance, however
[https://github.com/django/django/blob/ded485464214a3f69b64402b7d82221279f80008/django/db/models/options.py#L232
this is then deleted] whilst the `Options.contribute_to_class` class is
called. The attributes from the meta value are stored on a
`original_attrs` attribute however. It might be a good idea to document
this!!
=== Motivation for the documentation ===
The motivation for this came change is born out of a desire to write a
check that `db_table` is explicitly defined on all my models (even if it
matches the default value that django would apply). I can write checks
that require things like `ordering` is defined (or not defined) on a
model. The problem with `db_table` is that defaults are applied when
`contribute_to_class` is called. This means we need a reference to the
underlying original Meta data. The `original_attrs` attribute is perfect
for this, but as far as I can tell it isn't documented anywhere.
--
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Ticket URL: <https://code.djangoproject.com/ticket/35976#comment:3>
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