Using $prefix in Query.php

[spencerfinnell]

After noticing Query::set_prefix() I figured it would be a good idea to define a Query::$prefix property in my class that inherits the Query base. However, upon doing so I started seeing unexpected results.

After investigating it looks like it's possible to end up with shared cache groups because Base::apply_prefix() will see the ::$prefix property and then try to apply that to the empty passed string which comes from the default empty Query::$cache_group. Basically you get my-prefix- for every group name.

Explicitly defining the Query::$cache_group property resolved my issue but I'm wondering if there is a good place to document that both are needed if used, or if the parent class should throw an exception if there is a set prefix with no cache group?

[JJJ]

This variable detail definitely should be made better soon. It's pretty confusing.

The ::$prefix property is intended to be set once in the Base class for everything that extends it, and I did not plan on it being set differently by subclasses.

For Sugar Calendar the Base::prefix gets set to sc and for EDD it's edd.

Obviously though, this requires hard-coding a change into Berlin because you can't extend 2 classes at once. See #103.

Would love ideas on how to make this super easy.

[alexstandiford]

I wonder if simply using __set would be sufficient? Make it so that it can only be set once, perhaps?