#31278: Schema Editor Action: Pull Field Up
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Reporter: awiebe | Owner: nobody
Type: Bug | Status: new
Component: | Version: 3.0
Migrations | Keywords: pullup schemaeditor
Severity: Normal | runpython refactor inheritance
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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The schema editor is incapably of detecting when a field has been pulled-
up into the parent class, and instead tries to delete a pulled-up field,
and then add an equivalent field to the parent table, having deleted all
of the data that was in the original column.
At the very least the schema editor should ask whether you intended to do
a pull-up instead to prevent data loss. Below I will describe the
procedure for performing a pull up in an automatic way. Even if not
implemented as generated migrations, there should be an article on the
procedure, in the same way there is for other quirky runpython use-cases.
Instead the schema editor should generate two migrations ( it can't do it
atomically in one migration, or I couldn't figure out how to make that
work when I write this kind of migration manually).
==Migration 1==
1.Add the pulled up field to the parent class, permit null, default null
(so we don't have to set or consider any values)
2.RunPython Select all instances of derived objects using their default
manager (emit warning if more than one, or just to say the default is used
and edit the migration)
3.For each in (2) select, get the parent object through the _ptr field,
and copy the old field value to the new field
NB if multiple fields try to add them upfront, and copy every value at
once to prevent duplicate select queries.
==Migration 2==
Delete columns in child class, and proceed with any other schema changes
that would normally be generated.
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Ticket URL: <https://code.djangoproject.com/ticket/31278>
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