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Diffstat (limited to 'docs/ref')
| -rw-r--r-- | docs/ref/migration-operations.txt | 17 |
1 files changed, 17 insertions, 0 deletions
diff --git a/docs/ref/migration-operations.txt b/docs/ref/migration-operations.txt index 739b2bcdf8..bb5d451379 100644 --- a/docs/ref/migration-operations.txt +++ b/docs/ref/migration-operations.txt @@ -421,6 +421,8 @@ It accepts two list of operations, and when asked to apply state will use the state list, and when asked to apply changes to the database will use the database list. Do not use this operation unless you're very sure you know what you're doing. +.. _writing-your-own-migration-operation: + Writing your own ================ @@ -480,6 +482,21 @@ Some things to note: to them; these just represent the difference the ``state_forwards`` method would have applied, but are given to you for convenience and speed reasons. +* If you want to work with model classes or model instances from the + ``from_state`` argument in ``database_forwards()`` or + ``database_backwards()``, you must render model states using the + ``clear_delayed_apps_cache()`` method to make related models available:: + + def database_forwards(self, app_label, schema_editor, from_state, to_state): + # This operation should have access to all models. Ensure that all models are + # reloaded in case any are delayed. + from_state.clear_delayed_apps_cache() + ... + + .. versionadded:: 1.11 + + This requirement and the ``clear_delayed_apps_cache()`` method is new. + * ``to_state`` in the database_backwards method is the *older* state; that is, the one that will be the current state once the migration has finished reversing. |