Designing tag system that can tag multiple db tables

Question

Hi,

I am wanting to allow users to tag items so that they can search for them using tags. What is the best way of achieving this cleanly? So far the solution I have come up with only involves adding two extra tables to my current db system.

<db Trackable product 1>
int id;
info etc
</>

<db Trackable product 2>
int id;
info etc
</>

//defines the M:M relationship between Tag and various types of Trackable products
<db TagLink>
int trackableProd1Id
int trackableProd2Id
int tagId
</>

<db Tag>
int tagId
tag name etc
</>

Is this a good way to go about it? A benefit of this approach is that it should scale well and it also allows me to add more trackable products in the future by simply adding a column to TagLink table. This is obviously not a good idea if I planned to track 10's of tables but for up to 3-4 tables it should prove to work well shouldn't it?

Answer 1

Well, usually tags are implemented with many-to-many relationship (m:n relationship, if you want). There are three tables:

tags
    id INT NOT NULL AUTO INCREMENT
    name VARCHAR NOT NULL
    .
    .
    .
    possibly other fields
    .
    .
    .
    PRIMARY KEY (id)

items_you_want_to_tag
    id INT NOT NULL AUTO INCREMENT PRIMARY KEY
    name VARCHAR NOT NULL
    .
    .
    .
    possibly other fields
    .
    .
    .
    PRIMARY KEY (id)

xref
    tag_id INT NOT NULL
    item_id INT NOT NULL
    FOREIGN KEY (tag_id) REFERENCES tags(id)
    ON UPDATE CASCADE ON DELETE CASCADE,
    FOREIGN KEY (item_id) REFERENCES items_you_want_to_tag(id)
    ON UPDATE CASCADE ON DELETE CASCADE,
    PRIMARY KEY (tag_id, item_id)

The above schema is in pseudocode, of course.

So your schema looks just about right to me with one exception. If you want to tag two tables, I would create separate tags tables for each product type (Trackable product 1 and Trackable product 2 tables in your case) and also create two intersection tables. So you would have six tables.

Make sure to use correct indexes though, or else it won't scale that well :)

UPDATE:

Or, if you want to be able to tag both product types with the same tag, add another field to the intersection table containing a product group and add it to the multi primary key (as already pointed out by mjv ;)).

Answer 2

Rather than multiple "TrackableProd_N_id" columns in TagLink table, I suggest you introduce a multi-column foreign key, such as

   TagLink table
      int ProdGroup    -- "points" to table 1 vs. table 2 etc.
      int ProductId
      int TagId

in this fashion when additional sources of product come up you merely need to "invent" a new ProdGroup number for them, and use the ProductId (or other primary key from said table).