fix: allow adding discriminators using Schema.prototype.discriminator() to subdocuments after defining parent schema

[vkarpov15]

Fix #14109

Summary

For #14109, re: #13898. Delay applying discriminators added using Schema.prototype.discriminator() on embedded schemas (document arrays and single nested paths) until you compile the top-level model. This allows adding discriminators to document arrays using Schema.prototype.discriminator() after defining a document array with the schema, which is the primary issue in #14109.

Examples

[hasezoey]

Looks good to me, though there is one question:
the test define Class.type = number, but that field never seems to be used and does not match what is set in .discriminator(HERE, is this intentional?

[vkarpov15]

No, you're right that type doesn't do anything in the tests for #14109. I just copied that code from the person who reported the issue (adjusted for syntax). I removed the StringTyped.type = 4 etc. lines in 16e5308