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My application allows users to add tags to an image (similar to facebook). Each tag can consist of up to 8 attributes and there is currently no limit on the amount of tags a user can add.

At the moment, tag information is stored across 3 post meta key/value pairs. Each post meta value is serialized before storing.

We're approaching a rewrite of this functionality and I wanted to ask for some advice on the best approach to storing tag data.

IMO there are two approaches:

  1. post meta key/value for each tag/attribute (this has the potential to create a huge dataset - n tags times 8 attributes)

  2. A new table just for tags

Both approaches would avoid storing serialized data, which I think is what I need to achieve.

Which is the better approach?
Is a third option possibly even more feasible?

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Avoid serialized whenever you can. It is slow to read and you cannot really search for it.

Use custom tables – more than one:

  1. One for the relationship between users, tags and images: user_id, tag_id and image_id.
  2. One for tag meta data, your attributes, if I understand that right: tag_id, meta_1, meta_2

Now you can look up for each attachment ID if there is an entry for the user, and then look up the meta data from the meta table.

  • Thanks, I'll have a play around with a sample schema based on your suggestion – Oldie Apr 23 '13 at 11:11
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Rather than using a custom table or post meta to store the tag information you will probably find a simpler solution if you investigate the Custom Taxonomy support within WordPress.

You can register one (or more) custom taxonomies against the attachment post type.

The following Codex pages are a good place to start:

You may also find inspiration in existing WordPress plugins which provide similar functionality such as http://plugins.trac.wordpress.org/browser/matts-community-tags/trunk

  • I've had a play around with taxonomies and how I think it could work for tags with the application and I've got a couple of potential approaches. Feedback would be much appreciated. pastebin.com/79tnV1Ev – Oldie May 3 '13 at 15:07
  • @Oldie your pastebin link doesn't work for me – Peter Westwood May 7 '13 at 23:48
  • I've created a gist - gist.github.com/anonymous/dc2b058d7a36ae57c240 – Oldie May 8 '13 at 9:53
  • So looking at this the first approach is going to work more successfully because the second approach makes the data really hard to query using the WordPress APIs. – Peter Westwood May 8 '13 at 15:47
  • Thanks Peter, I've created a POC based around using custom taxonomies. Initial implementation looked very promising. However, I have noticed a potential issue. For example, tag_t1 can include the term 'Accessories'. The user may wish to add more than 1 look tag with the term 'Accessories' under the tag_t1 taxonomy (Accessories > belt, Accessories > glasses). From what I can see, it doesn't seem to be possible to add the same term more than once for a taxonomy. Can you confirm? Thanks. – Oldie May 15 '13 at 10:24

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