override columns with ForeignKey dupes existing foreign key

[Mike Bayer]

table = Table('mytable', meta,
   Column('id', Integer, ForeignKey('othertable.id'), primary_key=True),
   autoload=True
)

if the actual table has the same foreign key, youll get a "foreign keys" collection like

OrderedSet([ForeignKey(u'account_ids.account_id'),
ForeignKey('account_ids.account_id')](ForeignKey(u'account_ids.account_id'),
ForeignKey('account_ids.account_id')))

[jek]

this is related to #650. basically what is needed is a remove() that removes a column and its participation in all schema items, some of which reference the Column object and some of which are only by name at this point in autoload.

[Mike Bayer]

fbd0a0c725249ef22260a3714ed564a191c0c45c

Im wondering if reversing the order of operations simplfies this; load local Column objects first, then issue exclude_columns to the reflection process. Problem is, if a table is auto-reflected via a foreign key, then you cant redefine those columns (since the idea is to explicitly forbid "redefinition" of columns). Weirdly, issuing a Table constructor for an already-reflected table requires the "use_existing" flag, which is only documented in docstrings; id think we'd have 100s of irc and email queries about this but we don't...which implies that somehow, everyone whos using reflection somehow does not ever come across an auto-reflected table ?? that seems a little weird.

[Mike Bayer]

Removing milestone: 0.4.0 (automated comment)