Question

I have tables:

Articles{...}
Recipes{...}
Notifications{...}
Photos{...}

And I need to implement ‘user comments’ feature (like facebook).
Should I make tables: ArticleComments, RecipesComments etc. with 1:n relationship?
Or create one Comments table for all (but I have no idea how to design this)?

Answer 1

You could create another table CommentableEntity (although call it something better). Each of the rows in your tables (Articles, Recipes etc.) would have a reference to a unique row in this table. The entity table might have a type field to indicate the type of entity (to aid reverse joining).

You can then have a Comment table that references CommentableEntity, in a generic fashion.

So for example you’ll end up with the following tables:

Articles
-----------------
Article_id
CommentableEntity_id (fk, unique)
Content
....

Recipes
-----------------
Recipe_id
CommentableEntity_id (fk, unique)
Content
....

CommentableEntity
-----------------
CommentableEntity_id (pk)
EntityType (e.g. 'Recipe', 'Article')

Comment
-------
Comment_id (pk)
CommentableEntity_id (fk)
User_id (fk)
DateAdded
Comment 
...etc...

You can add the CommentableEntity record every time you add an Article/Recipe etc. All your comment-handling code has to know is the CommentableEntity_id – it doesn’t care what type of thing it is.