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I have a custom post type, Album, which has a custom field called 'tracklist'. Tracklist is a multidimensional array which contains a track title and track link, like so:

$tracklist = array(
  [0] => array(
    [title] => 'Stairway to Heaven'
    [link]  => 'stairway.mp3'
  [1] => array(
    [title] => 'Battle of Evermore'
    [link]  => 'battle.mp3'

I want to use get_posts to find all the albums with the song 'Stairway to Heaven.' Something like this (I know this won't work, but hopefully it will explain my intentions):

$albums = get_posts(array(
  'post_type'  => 'collection',
  'meta_query' => array(
      'key'   => 'tracklist',
      'value' => array(
        'key'   => 'track'
        'value' => array(
          'key'   => 'title',
          'value' => 'Starway to Heaven'

It's easy enough to use meta_query to look for a custom field that returns a single string, but in this case I'm looking a for a value buried a few levelsdeep. How would I do this?

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You can't, at least not efficiently. You best bet is to use the LIKE statement, but it would fail for searches like "title", "link", "mp3" :)

I suggest you build your own serialization method. For example, store the tracklist as a string like this:

Track1 title
Track2 title

Notice the new line character (\n). Use it to split the string into an array when you need to use the meta value.

Then you can have something like:

'meta_query' => array(
      'key'   => 'tracklist',
      'value' => 'Starway to Heaven',
      'compare' => 'LIKE',

The download links can go into a separate meta record.

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