articles, blog posts, photos, stories

I'm designing a database for a web site that will have at least 4 different object types represented (articles, blog posts, photos, stories), each of which have different enough data requirements to warrant their own tables. We want users to be able to post comments for any of these types. The data requirements for comments are simple and independent of the type of thing the comment regards (ie just a comment body, and the author's email).

I want to avoid the redundancy of creating and managing 4+ separate tables for the comments, so I'd like to be able to hold all comments in one table, possibly specifying the relation via 2 columns: one to designate the parent entity and one for the parent row Id.

but I don't understand how, then, I would implement foreign keys, since foreign keys establish a relation between 2 and only 2 tables (right?).

So with all that in mind, what would be the best approach?

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Here's one way to implement supertype/subtype tables for your app.

First, the supertype table. It contains all the columns common to all subtypes.

CREATE TABLE publications (
  pub_id INTEGER NOT NULL PRIMARY KEY,
  pub_type CHAR(1) CHECK (pub_type IN ('A', 'B', 'P', 'S')),
  pub_url VARCHAR(64) NOT NULL UNIQUE,
  CONSTRAINT publications_superkey UNIQUE (pub_id, pub_type)
);

Next, a couple of subtype tables.

CREATE TABLE articles (
  pub_id INTEGER NOT NULL,
  pub_type CHAR(1) DEFAULT 'A' CHECK (pub_type = 'A'),
  placeholder CHAR(1) NOT NULL, -- placeholder for other attributes of articles
  PRIMARY KEY (pub_id, pub_type),
  FOREIGN KEY (pub_id, pub_type) REFERENCES publications (pub_id, pub_type)
);

CREATE TABLE stories (
  pub_id INTEGER NOT NULL,
  pub_type CHAR(1) DEFAULT 'S' CHECK (pub_type = 'S'),
  placeholder CHAR(1) NOT NULL, -- placeholder for other attributes of stories
  PRIMARY KEY (pub_id, pub_type),
  FOREIGN KEY (pub_id, pub_type) REFERENCES publications (pub_id, pub_type)
);

The CHECK() and FOREIGN KEY constraints in these subtype tables prevent rows from referencing the wrong kind of row in the supertype. It effectively partitions the pub_id values among the subtypes, guaranteeing that any given pub_id can appear in one and only one of the subtype tables. That's why you need either a PRIMARY KEY or NOT NULL UNIQUE constraint on the pair of columns {publications.pub_id, publications.pub_type}.

The table for comments is simple. Given that it is to have the same structure for all subtypes, you can reference the supertype.

CREATE TABLE comments (
  pub_id INTEGER NOT NULL REFERENCES publications (pub_id),
  comment_timestamp TIMESTAMP NOT NULL DEFAULT now(),
  commenter_email VARCHAR(10) NOT NULL, -- Only allow people who have 
                                        -- really short email addresses
  comment_text VARCHAR(30) NOT NULL,    -- Keep 'em short!
  PRIMARY KEY (pub_id, comment_timestamp, commenter_email)
);

Add a little bit of data.

INSERT INTO publications VALUES
(1,'A', 'url 1 goes here'),
(2,'A', 'url 2 goes here'),
(3,'S', 'url 3 goes here');

INSERT INTO articles VALUES
(1,'A', 'A'),
(2,'A', 'B');

INSERT INTO stories VALUES
(3,'S', 'A');

INSERT INTO comments VALUES
(1, now(), 'a@b.com','You''re stupid'),
(1, now(), 'b@c.com', 'You''re stupid, too!');

Now you can create a view to show all articles and resolve the join. You'd do the same for each of the subtypes.

CREATE VIEW articles_all AS
SELECT P.*, A.placeholder
FROM publications P
INNER JOIN articles A ON (A.pub_id = P.pub_id)

You might prefer names like "published_articles" instead of "articles_all".

To select one article and all its comments, you can just left join the two tables. (But see below why you probably won't do that.)

SELECT A.*, C.*
FROM articles_all A
LEFT JOIN comments C ON (A.pub_id = C.pub_id)
WHERE A.pub_id = 1;

You'd probably not actually do that for a web interface, because the dbms would have to return 'n' copies of the article, where 'n' equals the number of comments. But it does make sense to do this in some applications. In applications where it makes sense, you'd use one updatable view for each subtype, and application code would use the updatable views most of the time.

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The more common business application of a supertype/subtype involves "Parties" (the supertype), "Organizations" and "Individuals" (the subtypes, informally companies and people. Addresses, like "comments" in the example above, are related to the supertype, because all the subtypes (organizations and individuals) have addresses.

---

You can use super-type/subtype in DB design to avoid that problem. Create a super-type for images, video, notes and then link to the super-type. Keep all common column in the super-type table.

Here are few links to several similar questions/answers with models:

One

Two

Three

Four

---

In my opinion you are better off having 4+ separate tables for the comments. Or you could have join tables. One table for all the comments... ex: blog table, comment table, blog_comment table. this would allow you to have your foreign keys.

Blog
--------
Blog_id
{other fields}

Blog_Comment
--------------
Blog_id
Comment_id


Comment
------------
Comment_id
{other fields}

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