Model Inheritance and select_for_update with 'of'
Consider two models A and B with B being a sub model of A. If I have a select and update that looks like this (assume it all happens inside an atomic transaction):
b = B.objects.filter(pk=1).select_for_update(of=('self'))
b = b[0] # Evaluate the query
b.some_field_of_b = b.some_field_of_b + 1
b.save()
This will cause race conditions. The generated SQL will only lock the table of A (the super-model) and not of B. As you can see, the field being updated is of B and not of A. Even though the SELECT .. FOR UPDATE OF might block, the queryset returned will not contain the updated data of the table of B because the snapshot will not include the updated data (before a lock is even acquired).
When the of parameter for select_for_update is supplied, the tables locked should include those of the sub-models of the of fields.
Change History
(15)
| Owner: |
changed from nobody to Abhijeet Viswa
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| Status: |
new → assigned
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| Keywords: |
select_for_update mti model inheritance added; race conditions psql concurrency db removed
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| Triage Stage: |
Unreviewed → Accepted
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| Version: |
2.2 → master
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| Cc: |
Abhijeet Viswa added
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| Has patch: |
set
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| Severity: |
Normal → Release blocker
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| Version: |
master → 2.1
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| Needs tests: |
set
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| Patch needs improvement: |
set
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| Needs tests: |
unset
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| Owner: |
changed from Abhijeet Viswa to Mariusz Felisiak
|
| Patch needs improvement: |
unset
|
| Resolution: |
→ fixed
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| Status: |
assigned → closed
|
I confirmed this is the case,
selfalways use the parent model table instead of the current one.I'm not convinced we should make
select_for_update(of=('self',))select all inherited tables though as that would prevents selecting only the current leaf model table. IMO the code should only be changed to makeselfpoint to the queryset's model table as documented.I suggest we also enhance the documentation to mention that parent links should be included in
ofwhen this is desired instead.In your case that would mean
of=('self', 'a_ptr')to select both tables for update.