在SQLAlchemy C中从多表联接中选择列

Question

我在SQLAlchemy Core中加入3个表并选择所有列，如下所示：

rows = self.db.execute(self.execs.join(
                         self.orders.join(self.instruments)
                      ).select(whereClause)).reduce_columns())

如果我想选择一个子集，但我想选择一个列：

^{pr2}$

它不工作，并给出以下错误：

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/apps/qtdist/pkgs/pub/cpython/python-2.7.6/lib/python2.7/site-packages/sqlalchemy/sql/selectable.py", line 807, in select
    return Select(collist, whereclause, from_obj=[self], **kwargs)
  File "/apps/qtdist/pkgs/pub/cpython/python-2.7.6/lib/python2.7/site-packages/sqlalchemy/sql/selectable.py", line 2219, in __init__
    whereclause).self_group(against=operators._asbool)
  File "/apps/qtdist/pkgs/pub/cpython/python-2.7.6/lib/python2.7/site-packages/sqlalchemy/sql/elements.py", line 3438, in _literal_as_text
    "SQL expression object or string expected."
sqlalchemy.exc.ArgumentError: SQL expression object or string expected.

另一种方法是使用select而不是加入。选择并使其与where子句隐式连接：

joinConditions = (orders.c.colx == execs.colx) & (execs.c.coly == instruments.c.coly)
select(reqdCols).where(and_(whereClause, joinConditions)

但是出于性能原因，我更喜欢显式连接而不是隐式连接。有没有任何方法可以使用显式联接来选择列的子集？在

Answer 1

具体做法如下：

j = join(table1, table2)   #optional third argument is join on part like table1.c.c1 == table2.c.c1

r = db.execute(select([table1.c.x, table2.c.y]).select_from(j)