What is the best way to link data in tables?

Question

MySQL

There are page models , blogs , photos .
How to properly arrange them to take with model comments ?
I have two options in mind.
1) In the comments model, add the fields page_id, blog_id, photo_id. In this case, it turns out that in the record there will be only one field with id, the rest will be NULL.
2) In each "parent" model, add the comment_id field. Here I have no idea how to implement the creation / saving.
Which one is better to use and are there any "pitfalls"?

Answer 1

Polymorphic link
Gems for comments
Or else - awesome-ruby.com
The pitfalls are that soon you will want to have comments for different entities.

Answer 2

1. It is best to connect a comment service, for example - HyperComments
2. If you still dare to make your bike, then I recommend making 3 tables!
I will explain why:
- comments for pages can contain a quote from the page or some other link. for blogs, this can also be - but for photos there is no such thing!
- comments for photos can have a rating from 0 to 10. and the page and blog may not.
- and there can be many such nuances, you are tormented to fill in 1/3 of the table. and 2/3 will be empty columns.
In short, these are different entities - and it is better to separate them into separate tables in the database. Then there will be no pitfalls!

Answer 3

Quite the first option will suit it (it will use the necessary columns in the right place), but I would probably still share it as suggested by Ivan Filatov