Not unique table/alias: 'priority' when priority enums are removed and priority custom field is added

[Jun Omae]

All of priority enums are removed and priority as custom field is added, query system raises an OperationalError on MySQL.

2016-03-14 12:03:35,208 Trac[util] DEBUG: SQL: SELECT COUNT(*) FROM (SELECT t.id AS
 id,t.summary AS summary,t.status AS status,t.type AS type,t.milestone AS
 milestone,t.component AS component,t.time AS time,t.changetime AS changetime,t.owner AS
 owner,priority.value AS priority_value,`priority`.value AS `priority`
FROM ticket AS t
  LEFT OUTER JOIN ticket_custom AS `priority` ON (`priority`.ticket=t.id AND `priority`.name='priority')
  LEFT OUTER JOIN enum AS priority ON (priority.type='priority' AND priority.name=priority)
WHERE ((COALESCE(t.status,'')!=%s) AND (COALESCE(t.owner,'')=%s))
ORDER BY COALESCE(priority.value,'')='',CAST(priority.value AS signed),t.id) AS x
2016-03-14 12:03:35,208 Trac[util] DEBUG: args: [u'closed', u'admin']
2016-03-14 12:03:35,208 Trac[util] DEBUG: execute exception: OperationalError(1066, "Not unique table/alias: 'priority'")

The issue doesn't raise on Trac 0.12.7 and MySQL.