collection cache key generates invalid SQL for relations with joins

[dpogue]

Steps to reproduce

Test case: https://gist.github.com/dpogue/f168bc6bc24848396bf6694623532b3a

Expected behavior

Rails should generate a cache key

Actual behavior

Rails generates a select statement that does not join the right tables, but still includes them in the where clause. This results in an invalid SQL statement that causes a database exception.

Workarounds

If you call #load on the relation before trying to generate a cache key, it is able to calculate the key on the data in memory, which avoids making any SQL statements.
Of course, that means now all your data has been loaded into memory, which is probably what you were trying to avoid by using caching.

System configuration

Rails version: 5.1.3 (test case is against master)

Ruby version: 2.3.3