1class Operation:
2 """
3 Base class for migration operations.
4
5 It's responsible for both mutating the in-memory model state
6 (see db/migrations/state.py) to represent what it performs, as well
7 as actually performing it against a live database.
8
9 Note that some operations won't modify memory state at all (e.g. data
10 copying operations), and some will need their modifications to be
11 optionally specified by the user (e.g. custom Python code snippets)
12
13 Due to the way this class deals with deconstruction, it should be
14 considered immutable.
15 """
16
17 # Can this migration be represented as SQL? (things like RunPython cannot)
18 reduces_to_sql = True
19
20 # Should this operation be forced as atomic even on backends with no
21 # DDL transaction support (i.e., does it have no DDL, like RunPython)
22 atomic = False
23
24 # Should this operation be considered safe to elide and optimize across?
25 elidable = False
26
27 serialization_expand_args = []
28
29 def __new__(cls, *args, **kwargs):
30 # We capture the arguments to make returning them trivial
31 self = object.__new__(cls)
32 self._constructor_args = (args, kwargs)
33 return self
34
35 def deconstruct(self):
36 """
37 Return a 3-tuple of class import path (or just name if it lives
38 under plain.models.migrations), positional arguments, and keyword
39 arguments.
40 """
41 return (
42 self.__class__.__name__,
43 self._constructor_args[0],
44 self._constructor_args[1],
45 )
46
47 def state_forwards(self, package_label, state):
48 """
49 Take the state from the previous migration, and mutate it
50 so that it matches what this migration would perform.
51 """
52 raise NotImplementedError(
53 "subclasses of Operation must provide a state_forwards() method"
54 )
55
56 def database_forwards(self, package_label, schema_editor, from_state, to_state):
57 """
58 Perform the mutation on the database schema in the normal
59 (forwards) direction.
60 """
61 raise NotImplementedError(
62 "subclasses of Operation must provide a database_forwards() method"
63 )
64
65 def describe(self):
66 """
67 Output a brief summary of what the action does.
68 """
69 return f"{self.__class__.__name__}: {self._constructor_args}"
70
71 @property
72 def migration_name_fragment(self):
73 """
74 A filename part suitable for automatically naming a migration
75 containing this operation, or None if not applicable.
76 """
77 return None
78
79 def references_model(self, name, package_label):
80 """
81 Return True if there is a chance this operation references the given
82 model name (as a string), with an app label for accuracy.
83
84 Used for optimization. If in doubt, return True;
85 returning a false positive will merely make the optimizer a little
86 less efficient, while returning a false negative may result in an
87 unusable optimized migration.
88 """
89 return True
90
91 def references_field(self, model_name, name, package_label):
92 """
93 Return True if there is a chance this operation references the given
94 field name, with an app label for accuracy.
95
96 Used for optimization. If in doubt, return True.
97 """
98 return self.references_model(model_name, package_label)
99
100 def allow_migrate_model(self, connection, model):
101 """Return whether or not a model may be migrated."""
102 if not model._meta.can_migrate(connection):
103 return False
104
105 return True
106
107 def reduce(self, operation, package_label):
108 """
109 Return either a list of operations the actual operation should be
110 replaced with or a boolean that indicates whether or not the specified
111 operation can be optimized across.
112 """
113 if self.elidable:
114 return [operation]
115 elif operation.elidable:
116 return [self]
117 return False
118
119 def __repr__(self):
120 return "<{} {}{}>".format(
121 self.__class__.__name__,
122 ", ".join(map(repr, self._constructor_args[0])),
123 ",".join(" {}={!r}".format(*x) for x in self._constructor_args[1].items()),
124 )