| | 1 | (I am attempting a shorter and clearer description of this bug, but leaving the original description intact below - carljm). |
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| | 3 | In Django 1.8, consider an app `A` with migrations `1` and `2` and a squashed migration `1_squashed_2` that replaces both `1` and `2`. Consider a deployment of this app which has only `1` applied. When it receives the update including `2` and `1_squashed_2` and is migrated, `2` is marked as applied in the database but `1_squashed_2` is not. |
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| | 5 | This does not immediately appear to be a problem, because as long as migrations `1` and `2` exist and `1_squashed_2` is marked as replacing them, Django automatically considers `1_squashed_2` to be applied (and shows it as such in `showmigrations`). But Django never actually records `1_squashed_2` itself as applied in the database. |
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| | 7 | At some point, once all deployments have migrated through `2`, the idea is that `1` and `2` can be removed, and the `replaces` tag removed from `1_squashed_2`. At this point we have a problem, because now Django considers `1_squashed_2` to not be applied, and tries to apply it again, causing errors because tables already exist, etc. The only solution is to manually `--fake` squashed migrations on all deployments when their `replaces` tag is removed, which is a pain and eliminates much of the convenience of the migration system; `--fake` should not be necessary in the normal course of things. |
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| | 9 | The solution appears simple: anytime a migration is marked as having been applied, Django should check if it is part of a replaced set, and if that replaced set is now fully applied, the replacing migration should also be marked as applied. |
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| | 11 | It may be that this check should be performed by `migrate` even when it hasn't applied any migrations in the current run, as that would help to correct the corruption already caused by this bug in many existing databases, but not yet noticed because the replaced migrations haven't yet been removed. (This correction should probably be mentioned to the user so it doesn't happen silently.) |
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| | 13 | Original report follows: |
| | 14 | |