Question

I’m a software developer. I love to code, but I hate databases… Currently, I’m creating a website on which a user will be allowed to mark an entity as liked (like in FB), tag it and comment.

I get stuck on database tables design for handling this functionality. Solution is trivial, if we can do this only for one type of thing (eg. photos). But I need to enable this for 5 different things (for now, but I also assume that this number can grow, as the whole service grows).

I found some similar questions here, but none of them have a satisfying answer, so I’m asking this question again.

The question is, how to properly, efficiently and elastically design the database, so that it can store comments for different tables, likes for different tables and tags for them. Some design pattern as answer will be best 😉

Detailed description:
I have a table User with some user data, and 3 more tables: Photo with photographs, Articles with articles, Places with places. I want to enable any logged user to:

• comment on any of those 3 tables

• mark any of them as liked

• tag any of them with some tag

• I also want to count the number of likes for every element and the number of times that particular tag was used.

1^st approach:

a) For tags, I will create a table Tag [TagId, tagName, tagCounter], then I will create many-to-many relationships tables for: Photo_has_tags, Place_has_tag, Article_has_tag.

b) The same counts for comments.

c) I will create a table LikedPhotos [idUser, idPhoto], LikedArticles[idUser, idArticle], LikedPlace [idUser, idPlace]. Number of likes will be calculated by queries (which, I assume is bad). And…

I really don’t like this design for the last part, it smells badly for me 😉

2^nd approach:

I will create a table ElementType [idType, TypeName == some table name] which will be populated by the administrator (me) with the names of tables that can be liked, commented or tagged. Then I will create tables:

a) LikedElement [idLike, idUser, idElementType, idLikedElement] and the same for Comments and Tags with the proper columns for each. Now, when I want to make a photo liked I will insert:

typeId = SELECT id FROM ElementType WHERE TypeName == 'Photo'
INSERT (user id, typeId, photoId)

and for places:

typeId = SELECT id FROM ElementType WHERE TypeName == 'Place'
INSERT (user id, typeId, placeId)

and so on… I think that the second approach is better, but I also feel like something is missing in this design as well…

At last, I also wonder which the best place to store counter for how many times the element was liked is. I can think of only two ways:

1. in element (Photo/Article/Place) table
2. by select count().

I hope that my explanation of the issue is more thorough now.

Answer 1

The most extensible solution is to have just one “base” table (connected to “likes”, tags and comments), and “inherit” all other tables from it. Adding a new kind of entity involves just adding a new “inherited” table – it then automatically plugs into the whole like/tag/comment machinery.

Entity-relationship term for this is “category” (see the ERwin Methods Guide, section: “Subtype Relationships”). The category symbol is:

Assuming a user can like multiple entities, a same tag can be used for more than one entity but a comment is entity-specific, your model could look like this:

---

BTW, there are roughly 3 ways to implement the “ER category”:

• All types in one table.
• All concrete types in separate tables.
• All concrete and abstract types in separate tables.

Unless you have very stringent performance requirements, the third approach is probably the best (meaning the physical tables match 1:1 the entities in the diagram above).