Developer-Guide.md

@@ -21,7 +21,7 @@
 ![Screen_Shot_2021-05-20_at_15.25.06](uploads/c1e22cca8768e103feabc3a59de78d65/Screen_Shot_2021-05-20_at_15.25.06.png)
 ![Screen_Shot_2021-05-20_at_15.25.08](uploads/d35bf27ba2d7c8796bf6cc52f4f607fb/Screen_Shot_2021-05-20_at_15.25.08.png)
 ![Screen_Shot_2021-05-20_at_15.25.11](uploads/073fff32652e4f7e8b7258724c2e121c/Screen_Shot_2021-05-20_at_15.25.11.png)
-Now, we can add some jobs that use the relays, URLs, and fetchers we just added:
+Now, we can add some jobs that use the relays, URLs, and fetchers we just added. The `url_id`, `relay_id`, and `target_fetcher` are [foreign keys](https://www.educative.io/blog/what-is-foreign-key-database) to the rows in the other tables:
 ![Screen_Shot_2021-05-20_at_15.25.56](uploads/8033947bae1eaf01ed6858a9208b36fc/Screen_Shot_2021-05-20_at_15.25.56.png)
 
 1. After a few seconds, one of the workers claims the job and processes it: