Let's suppose we have a page for creating blog posts with a form with text area transformed into an editor. That editor has properly configured custom upload mechanism meaning that it uses a controller function such as upload_file() and it also has a view which is only needed to run some javascript needed by CKEditor. Then let's also suppose we have a database table containing blog posts, another table containing information about uploaded files, and yet another table used for defining a many-to-many relation between posts and files. So every post can have files and every file might belong to several posts.
Now comes the tricky part. When we upload an arbitrary number of images, edit the post, and press submit button we'd want to indicate in the DB that the uploaded file belongs to the blog post which has just been created. This is useful to prevent accidental deletion of files still used by some of the blogs. Handling the blog post form and the upload separately so both the post and the files get into the right table is piece of cake. But how do we handle the intermediate table? How should we insert data about their relationship into the table? Problem arises from the fact that uploading the image and editing the post happen in different controllers and different views, so in this case we cannot just use forms and the DAL to make things work, or at least it's not a trivial task. Yep, you guessed right, the question is how should we do this?
