Commit bd3c2900 authored by Amos Onn's avatar Amos Onn Committed by Tim Graham
Browse files

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).
parent 5ccb7f5f

django/db/migrations/operations/special.py

+11 −7
Original line number Diff line number Diff line
@@ -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):