Modifying the Main Loop Query (wp_query->set)

Not only is bumping not allowed in these forums, it really does work against you. While it does have the effect of moving your topic near the top of the conventional list, most members in a position to help you use the “No Replies” filter. When you bump, your topic falls off of this list. And others not using the filter assume that someone is helping you because there is more than one post, so they skip over your topic.

Sorting queries by date strings is problematic and should be avoided. The posts are sorted alphabetically, where June 28 will appear before June 3 and both appear before May 1. Storing timestamps and ordering numerically works MUCH better. If you are stuck with date strings, there might still be hope. You use the ‘posts_orderby’ filter to alter the ORDER BY clause created by WordPress. You want it to use the mySQL CONVERT() function to change the date string to a datetime value.

A better approach if you’re stuck with date strings is to add another meta key that contains the actual timestamp equivalent of the date string. If you orderby that key, everything should work fine.

The errors you are getting are because you are calling alter_events_query() from the template page. This is unnecessary because you added the function as a callback to the ‘pre_get_posts’ actions. The action hook calls your function at the correct time to modify the query. Calling it again from the template is incorrect, the query has been run and cannot be altered at this point.

I’m not sure why alphabetic sorting wouldn’t work with dates in that format, though you would need to get the rest of the code correct for it to work 😉 Your first and second posts after my reply have this: $query->set('orderby', "start_date[0]");. ‘start_date’ appears to be an array reference due to the ‘[0]’, but it cannot be a PHP variable without the ‘$’. If you are trying to reference a value in an array, even with the $ it will not work because it is in quotes. Unless this is a column name? A rather unusual name, but maybe possible.

None of that really matters except for info on what not to do. The thing is you can only set 'orderby' to a very limited set of values listed in the Codex [[Class_Reference/WP_Query#Order_.26_Orderby_Parameters|WP_Query Reference]]. Your only possible choice here is 'meta_value'. Note that you must also set 'meta_key' for this to work. Setting 'orderby' to ‘start_date[0]’, ‘any_random_string’, or even ‘UNIX_TIMESTAMP(start_date)’ can never work, it has to be one of the arguments listed.

You don’t seem to quite understand ‘key’ arguments because in your last post you are trying set it to the result of UNIX_TIMESTAMP(). There should never be any need for this as ‘key’ values are always predetermined string values. ‘key’ and ‘meta_key’ are both set to the tag name under which meta data is stored and retrieved. For example, when you retrieve post meta, you might use this code:
$my_data = get_post_meta( $post->ID, 'my_tag_name');
‘my_tag_name’ is the same tag name you would assign as a value when setting various ‘key’ arguments:
$query->set('meta_key', 'my_tag_name');

Assuming the yyyy-mm-dd date string values are stored in postmeta under the key ‘start_time’, the following should result in alphabetic ordering by that date string:

$query->set('meta_key', 'start_time');
$query->set('orderby', 'meta_value' );

If the ordering doesn’t quite work out, you might also try ‘meta_value_num’ instead of ‘meta_value’ for the 'orderby' argument.

You can use 'meta_query' if need be, but it is not required to establish an order. In fact, there’s evidence that 'meta_query' could actually make ordering by a meta value more difficult.

If for any reason you do find the need to insert a mySQL function like UNIX_TIMESTAMP(), you now know you cannot pass it as a WP_Query argument. The way to insert mySQL functions is to hook into one of the various ‘posts_*’ filters and add it to the appropriate clause. Explaining this aspect is an entire How-To article in itself. You can perhaps gain better understanding of how to use these filters by examining the source code where the filters are applied. You would be most interested in ‘posts_orderby’ at line 3004. The results of all these filters are assembled at line 3189.

You should now have a better idea of what and what not to do. You may get your code working straight off. If you do not, I’ve found the ‘posts_request’ filter (Line 3192) very useful in debugging my WP_Query arguments. By hooking this filter, your callback is passed the final mySQL query string just before it is sent to mySQL. By using print_r() to output the query string, you can often examine it and discover exactly why the query is not working like you think it should.

I hope all of this gets you straightened out. Good luck!