There should never be SQL embedded in a model class. This should be moved to https://github.com/mozilla/treeherder-service/blob/master/treeherder/model/sql/reference.json. Use the key/value pair, `replace=[]`, to `execute()` to dynamically set the table name. I suspect the problem you're having with `read_host` is caused by the read host not being set in the datasource table.