Eloquentでリレーションを作成する方法 ref: http://qiita.com/asaokamei/items/ce87090c155d7b4d7f6d

file0.php

class Post extends Eloquent {
  public comments() {
    return $this->hasMany('Comment');
  }
}
class Comment extends Eloquent {
  public posts() {
    return $this->BelongsTo('Post');
  }
}

file1.php

$comment->posts()->associate($post);

file2.php

$post = $comment->post;

file3.php

$post->comments()->save($comment);
$post->comments()->saveMany($comments);

file4.php

$comments = $post->comments;