from django.db import router
class Operation:
"""
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)
Due to the way this class deals with deconstruction, it should be
considered immutable.
"""
# If this migration can be run in reverse.
# Some operations are impossible to reverse, like deleting data.
reversible = True
# Can this migration be represented as SQL? (things like RunPython cannot)
reduces_to_sql = True
# Should this operation be forced as atomic even on backends with no
# DDL transaction support (i.e., does it have no DDL, like RunPython)
atomic = False
# Should this operation be considered safe to elide and optimize across?
elidable = False
serialization_expand_args = []
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):
"""
Return 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):
"""
Take the state from the previous migration, and mutate it
so that it matches what this migration would perform.
"""
raise NotImplementedError(
"subclasses of Operation must provide a state_forwards() method"
)
def database_forwards(self, app_label, schema_editor, from_state, to_state):
"""
Perform the mutation on the database schema in the normal
(forwards) direction.
"""
raise NotImplementedError(
"subclasses of Operation must provide a database_forwards() method"
)
def database_backwards(self, app_label, schema_editor, from_state, to_state):
"""
Perform 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(
"subclasses of Operation must provide a database_backwards() method"
)
def describe(self):
"""
Output a brief summary of what the action does.
"""
return "%s: %s" % (self.__class__.__name__, self._constructor_args)
@property
def migration_name_fragment(self):
"""
A filename part suitable for automatically naming a migration
containing this operation, or None if not applicable.
"""
return None
def references_model(self, name, app_label):
"""
Return True if there is a chance this operation references the given
model name (as a string), with an app label for accuracy.
Used for optimization. If in doubt, return True;
returning a false positive will merely make the optimizer a little
less efficient, while returning a false negative may result in an
unusable optimized migration.
"""
return True
def references_field(self, model_name, name, app_label):
"""
Return True if there is a chance this operation references the given
field name, with an app label for accuracy.
Used for optimization. If in doubt, return True.
"""
return self.references_model(model_name, app_label)
def allow_migrate_model(self, connection_alias, model):
"""
Return whether or not a model may be migrated.
This is a thin wrapper around router.allow_migrate_model() that
preemptively rejects any proxy, swapped out, or unmanaged model.
"""
if not model._meta.can_migrate(connection_alias):
return False
return router.allow_migrate_model(connection_alias, model)
def reduce(self, operation, app_label):
"""
Return either a list of operations the actual operation should be
replaced with or a boolean that indicates whether or not the specified
operation can be optimized across.
"""
if self.elidable:
return [operation]
elif operation.elidable:
return [self]
return False
def __repr__(self):
return "<%s %s%s>" % (
self.__class__.__name__,
", ".join(map(repr, self._constructor_args[0])),
",".join(" %s=%r" % x for x in self._constructor_args[1].items()),
)