[Sqlalchemy-tickets] Issue #3559: Document bindparam usage in join condition docs (zzzeek/sqlalchemy)

[ctolsen <iss...@bi...>]

New issue 3559: Document bindparam usage in join condition docs
https://bitbucket.org/zzzeek/sqlalchemy/issues/3559/document-bindparam-usage-in-join-condition

ctolsen:

I've been using sqlalchemy for a fairly long time, but never touched upon using the `bindparam` concept in a join condition. It's an incredibly useful concept that saves me a lot of headaches, and I'm sad I didn't discover it before.

Maybe it could be documented at more accessible stage, somewhere around [here](http://docs.sqlalchemy.org/en/rel_1_0/orm/join_conditions.html#specifying-alternate-join-conditions)? A use case where you can specify a bindparam for `Address.city` instead of using the static `Boston` value would be great.

I had a stab at writing an update, but I'm a bit out of water unfortunately.