You can’t. You are joining two keyspaces on a condition, you’re simply collating all joined documents into a field when using NEST. An index is required (in the version used) to support the join.
The only exception to requiring an index for this is if the right target’s key(s) could be identified by a field in the left target - if you were selecting purchases and nesting product here you could use the ON KEYS clause to bypass the need for an index.