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Fixed #25892 -- Optimized SeparateDatabaseAndState.database_backwards(). 

Now intermediate states in the database_backwards are cached, similar to
the executor's migrate() (or _migrate_all_backwards()). This makes the
migration run much faster (O(n) instead of O(n^2) over number of
database_operations).
This commit is contained in:
Amos Onn 2015-12-08 17:18:00 +02:00 committed by Tim Graham
parent 5ccb7f5f22
commit bd3c2900fc
1 changed files with 11 additions and 7 deletions
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16
django/db/migrations/operations/special.py
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@ -45,13 +45,17 @@ class SeparateDatabaseAndState(Operation):
def database_backwards(self, app_label, schema_editor, from_state, to_state):
# We calculate state separately in here since our state functions aren't useful
base_state = to_state
for pos, database_operation in enumerate(reversed(self.database_operations)):
to_state = base_state.clone()
for dbop in self.database_operations[:-(pos + 1)]:
to_states = {}
for dbop in self.database_operations:
to_states[dbop] = to_state
to_state = to_state.clone()
dbop.state_forwards(app_label, to_state)
from_state = to_state.clone()
database_operation.state_forwards(app_label, from_state)
# to_state now has the states of all the database_operations applied
# which is the from_state for the backwards migration of the last
# operation.
for database_operation in reversed(self.database_operations):
from_state = to_state
to_state = to_states[database_operation]
database_operation.database_backwards(app_label, schema_editor, from_state, to_state)
def describe(self):