Typical Multifeed Gotchas

Being able to merge the items from multiple feeds together simply and easily is really cool, but it also comes with some caveats that you need to watch out for.

One of the (very) common recurring threads we get in support is “I'm using Multifeeds, but the feeds are sorting by [insert blame here] instead of the date!” SimplePie IS, in fact, sorting by date, but in 100% of cases, a person is trying to merge in one or more feeds where the items don't have a date stamp.

Internally, SimplePie uses PHP's usort() function to compare the values of $item→get_date('U') for each item. If an item has a non-existent or invalid date stamp, SimplePie doesn't bother to fire the sorting function because the data isn't going to come out right anyway.

Remove the feed (or feeds) that doesn't have dates for all of its items from the list of feeds to process.

A bug report has been filed to add additional logic to sort these unsorted items to the end of the array instead of the beginning. This is the ideal solution, but it doesn't exist yet. Add a patch to the aforementioned bug report if you take the time to do this yourself.

You're merging multiple feeds together and you try to access data from $feed→get_title() or $feed→get_description() but there doesn't seem to be anything. It's because when you're merging multiple feeds together, there is no single feed for $feed to contain data for.

Let's say that you're merging together 3 feeds, each with their own titles, descriptions, etc. We'll use Digg, Slashdot, and Apple as examples. Digg has its own title, so does Slashdot, and so does Apple. If there are 3 competing pieces of data, what should $feed→get_title() return?

Well, put simply, SimplePie has no idea which data to show for $feed→anything(), so it returns null.

The safest (and most consistent) way to always ensure that you can access feed-level data from within the context of an item (e.g. such as inside a for() or foreach() loop), is to access the item's parent data using get_feed().

// Only works when you're handling a single feed. Has no value when using Multifeeds.
$feed->get_title();
 
// Always works, assuming you've defined $item as a feed item (such as in a foreach() loop).
$item->get_feed()->get_title();

$item→get_feed() is a reference to that item's parent $feed object, which contains all of the data that is available for that feed. Use this instead of a plain $feed when you're using Multifeeds.

As an example of something stupid and ridiculous that you could, but should never do:

echo $feed->get_item(0)->get_feed()->get_item(0)->get_feed()->get_item(0)->get_feed()->get_item(0)->get_feed()->get_item(0)->get_feed()->get_item(0)->get_feed()->get_title();

It's also always good practice to check to see if a method has a value before trying to output it:

if ($feed->get_title())
{
	echo $feed->get_title();
}

For reasons outlined above, $feed→data doesn't exist, so if you're used to checking for it before doing anything, it'll return false every time.

This issue is explained in greater detail here: I'm getting memory leaks!.