How can I import an old pgdump into an updated schema? I'm DB newb doing a software upgrade, and my predecessor's original migration plan may not have been as well thought out as I thought...

The Plan was to use our backup, created via

pg_dump -U derp -h db-host -Fc derp &gt; backup


Flash a clean VM with an upgraded software, with its fancy new schema and use 

pg_restore -v -U derp -n public -c -1 -h db-host -d derp backup


to load it with all its fancy old data.

Unfortunately I'm getting errors because of the updated schema, with new columns that create new relations.  "cannot drop constrain X on table Y because other objects depend on it"

I tried using -a (data only) but that fails fast because there are many lookup tables with now duplicated keys.

Can I somehow import the existing copy?

If not, can I alter the new database schema in a way that will allow my old dump to work?

If not.... can I make a different, more useful copy of the old database?

If I'm asking really dumb questions, what should I be asking?
It sounds you are dealing with some 3rd party's application.  In other words, the app vendor and your predecessor are not the same person.

The 3rd party would generally provide you with two things, an installer to use for new customers, and an upgrade process to upgrade the schema between versions of their app for existing customers.  You either need to use one, or the other, not some bastardized combination between them.  Restoring a dump of the old schema+data over the top of a newly-initialized schema for new customers would not be expected to work, unless the vendor said it does.  

The vendor might also provide instructions on how to upgrade your PostgreSQL version, either by itself or coupled with upgrading the application version.  If they do, then find and follow those instructions.  If they don't then you have a few options.


Go back to using the old database.  Run the upgrade scripts against it.  Worry about migrating that to a new database server later.
Create an empty database cluster (completely empty, not using the vendor's new-customer initialization stuff) and restore to that database cluster.  The easiest way to do that is probably to use the "-C" (note upper case) flag to pg_restore.  This will create the new database for you, with the name, encoding, etc. which was recorded in the dmp file.  Alternatively, perhaps you just use your existing cluster (initialized with the app vendors new-customer schema) and add the -C flag to your restore.  What that will do is drop the database they provided in-bulk (which will get around the dependency problems) and recreate it from what is in the dump file, meaning you get the old schema not the new one.  Either way, you will now have the old application schema running in a new PostgreSQL database.  Once this is working, then run the application upgrade script against this.
Ask the 3rd party app vendor what to do.