Fix test migration crash waiting to happen due to inferred relations Avoid crashing when a relation definition which is already known (due to being inferred) gets explicitly added. As explained in a comment, this would deserve more thinking, but at least this fixes a test failure with computed relations.

if repo.system_source.dbdriver == 'postgres':
    sql('ALTER TABLE appears ADD COLUMN weight float')
    sql('UPDATE appears SET weight=1.0 ')