[web2py] IntegrityError('FOREIGN KEY constraint failed',)

*WHEN I TRY ENTERING A STUDENT'S DETAILS AND SELECTING THEIR GUARDIAN AS FROM THE DROP DOWN MENU AS REFERNCED I GET A FOREIGN CONSTRAINT FAILED ERROR MESSAGEMY TABLES* db.define_table('guardian', Field('surname', requires=IS_NOT_EMPTY()), Field('name',

[web2py] IntegrityError: foreign key constraint failed

Hello I do not know much English I have two tables: db.define_table('category', Field('name')) db.define_table('books', Field('title', requires=IS_NOT_EMPTY('Введите название книги'), label='Название*'), Field('author', requires=IS_NOT_EMPTY('Введите автора'),