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51 lines
2 KiB
Python
51 lines
2 KiB
Python
#
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# db.py -- Database utilities.
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#
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# Copyright (c) 2007-2009 David Trowbridge
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#
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# Permission is hereby granted, free of charge, to any person obtaining
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# a copy of this software and associated documentation files (the
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# "Software"), to deal in the Software without restriction, including
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# without limitation the rights to use, copy, modify, merge, publish,
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# distribute, sublicense, and/or sell copies of the Software, and to
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# permit persons to whom the Software is furnished to do so, subject to
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# the following conditions:
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#
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# The above copyright notice and this permission notice shall be included
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# in all copies or substantial portions of the Software.
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#
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# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
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# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
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# IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
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# CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
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# TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
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# SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
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#
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from django.db import models, IntegrityError
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class ConcurrencyManager(models.Manager):
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"""
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A class designed to work around database concurrency issues.
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"""
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def get_or_create(self, **kwargs):
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"""
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A wrapper around get_or_create that makes a final attempt to get
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the object if the creation fails.
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This helps with race conditions in the database where, between the
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original get() and the create(), another process created the object,
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causing us to fail. We'll then execute a get().
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This is still prone to race conditions, but they're even more rare.
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A delete() would have to happen before the unexpected create() but
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before the get().
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"""
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try:
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return super(ConcurrencyManager, self).get_or_create(**kwargs)
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except IntegrityError:
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kwargs.pop('defaults', None)
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return self.get(**kwargs)
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