[Buildbot] #3088: add a foreign key from builds to buildslaves

[Buildbot trac]

#3088: add a foreign key from builds to buildslaves
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Reporter:  dustin       |       Owner:  rutsky
    Type:  enhancement  |      Status:  assigned
Priority:  major        |   Milestone:  0.9.0
 Version:               |  Resolution:
Keywords:  database     |
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Comment (by rutsky):

 I'm doing this during database migration in #2340.

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Ticket URL: <http://trac.buildbot.net/ticket/3088#comment:2>
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