One of the things that’s nice bout working with post types – custom or standard – in WordPress is that it’s really easy to hook into the serialization process in order to handle the data. This means that we have the ability to sanitize, format, read, access, modify, verify, etc. all of the data with the post type and with the post type’s meta data before it’s written to the database.

WordPress has a pretty consistent way of displaying error messages throughout the application. Really, it’s pretty consistent in how it displays all types of messages – success, updates, and errors – throughout the  system.

Let’s say that you’re working with a WordPress meta box, several of its fields are required, and you want to:

  • verify the input has been specified
  • either display an error message if it’s not specified
  • or write the data to the database if it checks out

The serialization process is pretty standard stuff, but if you’re looking to make sure required fields aren’t empty and that a error message is displayed whenever they’re not entered, then you’ll need to do some additional work.

Post Meta Data Error Messages

First, I want to mention that I know it’s possible to write code that will double-check the presence of inputs and then will render the error messages appropriately within the context of the meta box, but I dislike this for two reasons:

  1. It hijacks the experience that users are used to having within the Dashboard. That is, it displays messages where they aren’t normally displayed.
  2. If the meta box is located too low down the page and the message is displayed in said area, then the user may not see it at all.

Yes, the second point is really more of a consequence of the first but it’s still something worth considering especially since we’re using displays that range from, say, 11″ to 21″ (or more). And that’s the rationale behind why the following code does not display the errors in the meta box, but in the standard messaging area.

Location of the Death Star

Location of the Death Star

Anyway, for the purposes of this example, we’ll be looking at a Location post type and meta data for the death_star_plans.

1. Save the Location

First, we need to hook into the save_post action so that we can work with the meta data that’s associated with the post. For this example, assume that we’re working with an location post type and a piece of associated post meta data, say, death_star_plans.

The code above (as with the rest of the code in this post) is heavily commented in order to make it easy to follow, but to make sure we’re on the same page: In the gist, we’re hooking into the save process, verifying that we’re working our location post type and that the user has permission to save the data.

If not, then we stop the function from executing.

If so, then we proceed by calling a function responsible for evaluating the presence of the meta data and handling whether or not it should be saved or an error should be displayed.

2. Verify the Data Exists

As seen in the gist above, we need to check to see if the meta data exists. If so, then we’ll save it; otherwise, we need to display an error message:

Note that this could can be as simple or as complex as you’d like. Obviously, I’m just checking to see if the value in the $_POST array is set; however, there may be additional checks to add (such as if it’s of a certain length, if it’s only a number, etc.) depending on the type of meta data you’re saving.

3. Save the Plans

If we hit this point, then the presence of the data has passed so we can go ahead and save this to the database:

As commented, I’m only doing a very base-level of sanitization in this post. Again, this can be as complex as you like, but I’ve kept it simple for the purpose of this post.

4. Add the Errors

On the other hand, if the Plan meta data is not specified, then we need to add an error message to the list of error messages to be displayed on the post editor dashboard:

Here, we’re adding the data to WordPress’ settings error message collection. Since we’re not actually on a settings page, but on a post editor page, we need to set a transient so that WordPress will pick it up when it checks for messages. Finally, we remove this function from the admin_notice hook so it doesn’t fire again.

5. Display the Errors

Finally, if the errors exist, then we need to display them:

To do this, we simply check the transient value. If it doesn’t exist, then we stop the function; otherwise, we build up the list of errors, write it out to the display, and clear them so they aren’t displayed more than once.

The How and Why

As mentioned several times through this post, this code can be as simple or complex as you’d like. It’s not so much about what the code is doing. Instead, it’s about how to actually go about doing this in a clean, maintainable way.

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