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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
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 __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()),
)
def __eq__(self, other):
return (self.__class__ == other.__class__) and (self.deconstruct() == other.deconstruct())
def __ne__(self, other):
return not (self == other)
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