Postgres.app upgrade, now Rails app won't start

Question 1

I found a solution that works for me and requires minimal hacking/configuring. You only need to do this once and it will work for every bundle install. Add the following to your .bash_profile, .bash_rc, or equivalent:

export DYLD_FALLBACK_LIBRARY_PATH=/Applications/Postgres.app/Contents/MacOS/lib:$DYLD_LIBRARY_PATH

(Assuming you installed Postgres.app in the default location). Then restart your terminal session and try again.

Exporting to DYLD_LIBRARY_PATH directly can cause serious problems with other apps that depend on it, but using the fallback path avoids these problems.

See also:

EDIT: It seems that setting DYLD_FALLBACK_LIBRARY_PATH causes an error when you try to run psql. To fix this, you can add the following two lines to your .bash_profile:

alias psql="(. ~/.bash_profile; unset DYLD_FALLBACK_LIBRARY_PATH; psql)";

This is assuming that you're using bash and that your .bash_profile is located in your home directory. If that's not the case (or if you're using a .bashrc or other environment setup instead of .bash_profile) change the ~/.bash_profile part of the command to the path to your environment setup script.

The aliased commands basically start a subshell which does not effect your current bash environment. So when it unsets the DYLD_FALLBACK_LIBRARY_PATH variable, it's only temporary. After you exit psql the environment variable will be set again so that rails functions properly.

Question 2

It's likely your pg gem in your app was built against the old libraries. Try rebuilding it against the new Postgres.app:

$ gem uninstall pg
[...]
$ bundle install
[...]
"installing pg" (or something..)