#25068: Metaclass conflict when doing createmigrations in ModelState.render
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Reporter: kosz85 | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: 1.8
Severity: Normal | Resolution:
Keywords: metaclass conflict | Triage Stage: Accepted
createmigrations |
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 1
Easy pickings: 0 | UI/UX: 0
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Comment (by kosz85):
That's not smth to be scared of. I can explain what is happening there:
Each class has metaclass, which default is `type`. It's class of class.
You can change it and made your own type declaring metaclass. What new
migrations did, is bad assumption that class would be created only using
`type`, and that there would be only simple conflicts.
First take a note that `Model` metaclass is already set to `BaseModel`.
Assume such situation:
class cA with metaclass mA
class cB with metaclass mB
class cC(cA, cB) -> metaclass mC
Python has no problem with standard metaclasses.
When mA and mB eguals default `type` then python knows that mC is also
`type`.
Also when mA or mB equals default `type` and the other not (for example
`BaseModel`) then he can easily solve conflict because every metaclass is
derived from `type` so Python is using the other one so mC would be
`BaseModel`.
But when mA is custom and mB is custom, he don't know what to do, and
there are even worse scenarios, there may be 3 or more bases to inherit
from. Then standard response is to manually build mC that inherit from mA
and mB (like in normal class inheritance).
So what is doing this patch is reacting to that `TypeError`, and
constructing (it's normal factory) such custom metaclass from metaclasses
that are used in bases of this class. That new metaclass is used then to
create object, instead of standard type. So no magic ;) The only magic is
factory and populating metaclasses from bases.
--
Ticket URL: <https://code.djangoproject.com/ticket/25068#comment:21>
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