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Because your new category field is mandatory (you can’t create a book or bookmark without giving it a category), south asked you what to do with the existing books and bookmarks in your database – your latest changes would have made all your current books / bookmarks invalid (since they don’t have a category).
Your new category field is represented in your book/bookmark tables using a primary key of the category table. This primary key will likely be an integer (or possibly a string).
South asked you to supply a default primary key. Instead of supplying the primary key of an category object in your database (which would be an integer or a string). You’ve supplied a datatime object.
It’s crapping out when it actually runs the migration because the field doesn’t hold datetime objects. It holds integers (or strings). you need to create the migration again and add a valid primary key.