[sqlalchemy] use "USING ( join_column [, ...] )" instead of "ON join_condition"

[Manlio Perillo]

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Hi.

When selecting against joined tables, SQLAlchemy always use the
"ON join_condition" form.

The problem with this is that when the joined columns have the same
name, they are included 2 times in the result set.


Example (PostgreSQL):

CREATE TABLE foo (
    id TEXT PRIMARY KEY,
    x INTEGER
);

CREATE TABLE bar (
    a INTEGER PRIMARY KEY,
    id TEXT REFERENCES (foo.id) NOT NULL
);

INSERT INTO foo VALUES ('ID', 777);
INSERT INTO bar VALUES (0, 'ID');

SELECT * FROM foo JOIN bar ON foo.id = bar.id;
 id |  x  | a | id
- ----+-----+---+----
 ID | 777 | 0 | ID


If we use "USING ( join_column [, ...] )", instead, only one of each
pair of equivalent columns will be included in the join output, not both.


SELECT * FROM foo JOIN bar USING (id);
 id |  x  | a
- ----+-----+---
 ID | 777 | 0


Is it possible to change SQLAlchemy to use the second form when the
joined columns have the same name?

In alternative, this can be requested explicitly, as example:

class Join(FromClause):
    __visit_name__ = 'join'

    def __init__(self, left, right, onclause=None, using=None,
                 isouter=False):
        ...

When specified ``using`' take precedence over `onclause`.



Thanks   Manlio Perillo
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