A foreign key is used in relational databases to connect records between tables. It ensures that a value in one table refers to a valid record in another table.

For example, a BlogPosts table may contain a CategoryId field that references the Id field in the Categories table. This prevents blog posts from being assigned to non-existing categories.

Delete behavior is also important in foreign key relationships. Cascade delete removes child records automatically when the parent is deleted. Restrict prevents deleting the parent if child records exist.

In CMS projects, Restrict is often preferred between Category and BlogPost because deleting a category should not accidentally delete all blog posts. On the other hand, Cascade can be appropriate between BlogPost and BlogPostTranslation because translations do not make sense without the main post.