A PRIMARY KEY can be a multiple-column index. However, you cannot create a multiple-column index using the PRIMARY KEY key attribute in a column specification. Doing so only marks that single column as primary. You must use a separate PRIMARY KEY(index_col_name, ...) clause.

And

CREATE TABLE `ask_interest`
(
        `question_id` INTEGER  NOT NULL,
        `user_id` INTEGER  NOT NULL,
        `created_at` DATETIME,
        PRIMARY KEY (`question_id`,`user_id`),
        CONSTRAINT `ask_interest_FK_1`
                FOREIGN KEY (`question_id`)
                REFERENCES `ask_question` (`id`),
        INDEX `ask_interest_FI_2` (`user_id`),
        CONSTRAINT `ask_interest_FK_2`
                FOREIGN KEY (`user_id`)
                REFERENCES `ask_user` (`id`)
)Type=MyISAM;