from __future__ import unicode_literals from django.db import router 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) 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): """ 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('subclasses of Operation must provide a state_forwards() method') 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('subclasses of Operation must provide a database_forwards() method') 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('subclasses of Operation must provide a database_backwards() method') def describe(self): """ Outputs a brief summary of what the action does. """ return "%s: %s" % (self.__class__.__name__, self._constructor_args) def references_model(self, name, app_label=None): """ Returns True if there is a chance this operation references the given model name (as a string), with an optional 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=None): """ Returns True if there is a chance this operation references the given field name, with an optional 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): """ Returns if we're allowed to migrate the model. 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, in_between, app_label=None): """ 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()), )