Re: [Sqlalchemy-tickets] [sqlalchemy] #2722: clarify what should happen with query(A).select_from(A).join(B.a)

[sqlalchemy <mi...@zz...>]

#2722: clarify what should happen with query(A).select_from(A).join(B.a)
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      Reporter:  rbu            |      Owner:  zzzeek
          Type:  defect         |     Status:  new
      Priority:  medium         |  Milestone:  0.8.xx
     Component:  documentation  |   Severity:  major - 1-3 hours
    Resolution:                 |   Keywords:
Progress State:  in queue       |
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Comment (by zzzeek):

 yeah I've looked into seeing how an expression like:

 {{{
 session.query(A).select_from(A).join(B.a)
 }}}

 could establish the join as "A to B", but this would be an awkward
 exception case added to what is otherwise becoming a fairly consistent
 system.  Using the relationship for the join, "B.a", means "join from B to
 A", and that takes precedence here.   This works well because "B.a"
 contains both the two endpoints, the ON clause, and the direction, i.e.
 which side is left and which is right.   If Query were to second-guess
 this, and look at "select from", that becomes more complicated, in the
 case where we are already "selecting from" many things, such as
 `query(A).select_from(A).join(A.cs).join(C.bs)` etc. - it means we have to
 look at each of "A", "C", and "B" and make a guess which is the best "left
 side" to use.

 I do sometimes have the case where I don't have a backref set up and would
 like to use the reverse side, so I'm wondering if a new symbol like
 `session.query(A).join(B.a.reversed)` might be possible.

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Ticket URL: <http://www.sqlalchemy.org/trac/ticket/2722#comment:5>
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