django1/django/db/migrations/operations/base.py

63 lines
2.1 KiB
Python

class Operation(object):
"""
Base class for migration operations.
It's responsible for both mutating the in-memory model state
(see db/migrations/state.py) to represent what it performs, as well
as actually performing it against a live database.
Note that some operations won't modify memory state at all (e.g. data
copying operations), and some will need their modifications to be
optionally specified by the user (e.g. custom Python code snippets)
"""
# If this migration can be run in reverse.
# Some operations are impossible to reverse, like deleting data.
reversible = True
def __new__(cls, *args, **kwargs):
# We capture the arguments to make returning them trivial
self = object.__new__(cls)
self._constructor_args = (args, kwargs)
return self
def deconstruct(self):
"""
Returns a 3-tuple of class import path (or just name if it lives
under django.db.migrations), positional arguments, and keyword
arguments.
"""
return (
self.__class__.__name__,
self._constructor_args[0],
self._constructor_args[1],
)
def state_forwards(self, app_label, state):
"""
Takes the state from the previous migration, and mutates it
so that it matches what this migration would perform.
"""
raise NotImplementedError()
def database_forwards(self, app_label, schema_editor, from_state, to_state):
"""
Performs the mutation on the database schema in the normal
(forwards) direction.
"""
raise NotImplementedError()
def database_backwards(self, app_label, schema_editor, from_state, to_state):
"""
Performs the mutation on the database schema in the reverse
direction - e.g. if this were CreateModel, it would in fact
drop the model's table.
"""
raise NotImplementedError()
def describe(self):
"""
Outputs a brief summary of what the action does.
"""
return "%s: %s" % (self.__class__.__name__, self._constructor_args)