Add support for migrating data between database installations

[luislew]

Issue by luislew
Monday Mar 12, 2018 at 20:35 GMT
Originally opened as https://github.com/NerdWallet/nwpy-savage/issues/10

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Currently, version_id is generated from txid_current(), which is not portable between database installations. As a result, it's not possible to simply dump data from one server to another, because the version ID generation may conflict.

One way to around this issue would be to add a database_version_id, which would be incremented upon a database migration. SavageLogMixin would need to add a database_version_id column, and there would need to be a separate DatabaseVersionId table with a single value (which defaults to 0/1).