This makes the comments and the test more comprehensible. sti looks like a typo STI looks like an abbreviation.

Since we're here we also might consider changing the sentence:

    # superclass. This is mostly useful in relation to single-table
    # inheritance structures where you want a subclass to appear as the
    # superclass.

To include (STI) and remove the hyphenation of single-table:

    # superclass. This is mostly useful in relation to single table
    # inheritance (STI) structures where you want a subclass to appear as the
    # superclass.

We're also referring to it as "single-table" inheritance, so we might want to remove the quotes.