Thomas M Steenholdt daba67611b Enable pool_pre_ping in DB connection
To avoid problems with inactive DB connections, SQLAlchemy provides a `pool_pre_ping` option, that described in more detail here:

http://docs.sqlalchemy.org/en/latest/core/pooling.html#disconnect-handling-pessimistic

In flask environments, it's enabled by subclassing SQLAlchemy, which is what I've done here.

Fixes errors like:
sqlalchemy.exc.OperationalError: (_mysql_exceptions.OperationalError) (2006, 'MySQL server has gone away') which results in an Error 500 in the UI.
2018-06-12 14:01:25 -02:00
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2018-06-08 13:26:06 +02:00