39 lines
1.4 KiB
Python
39 lines
1.4 KiB
Python
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class Operation(object):
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"""
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Base class for migration operations.
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It's responsible for both mutating the in-memory model state
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(see db/migrations/state.py) to represent what it performs, as well
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as actually performing it against a live database.
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Note that some operations won't modify memory state at all (e.g. data
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copying operations), and some will need their modifications to be
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optionally specified by the user (e.g. custom Python code snippets)
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"""
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# If this migration can be run in reverse.
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# Some operations are impossible to reverse, like deleting data.
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reversible = True
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def state_forwards(self, app, state):
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"""
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Takes the state from the previous migration, and mutates it
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so that it matches what this migration would perform.
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"""
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raise NotImplementedError()
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def database_forwards(self, app, schema_editor, from_state, to_state):
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"""
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Performs the mutation on the database schema in the normal
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(forwards) direction.
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"""
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raise NotImplementedError()
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def database_backwards(self, app, schema_editor, from_state, to_state):
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"""
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Performs the mutation on the database schema in the reverse
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direction - e.g. if this were CreateModel, it would in fact
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drop the model's table.
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"""
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raise NotImplementedError()
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