Make a start on operations and state (not sure if final layout)

django/db/migrations/operations/base.py

@@ -0,0 +1,38 @@
+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 state_forwards(self, app, 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, 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, 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()