각각의 relationship을 정의해 보죠..  참조 : 여기

$table->foreign('group_id')->references('id')->on('groups');

$table->foreign('member_id')->references('id')->on('users');

$table->foreign('board_id')->references('id')->on('boards');

$table->foreign('member_id')->references('id')->on('users');

$table->foreign('board_id')->references('id')->on('boards');

$table->foreign('post_id')->references('id')->on('posts');