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Populate a Form Dropdown from the Database

Sometimes, you want to give the user the ability to select from a list if things that are in the database, rather than a fixed set of options as defined in the field definition. This will require some simple PHP along with a database query, but once you get the idea, a lot of other possibilities will become available to you.

Our Example

So, for this tutorial, let’s say you have a signup form for volunteers. Your volunteers fill out the form and indicate that they are willing to volunteer their time for a project. Let’s say you also have volunteer coordinators that have a different signup form where they can select a volunteer to assign to the project. (I’ve explained how to set up different signup forms for different kinds of people in another tutorial.)

When the volunteer coordinator signs up, you will want to provide them with a dropdown selector that lists all of the volunteers. The only way to do that is to get the list of volunteers from the database and then use that to populate the dropdown.

The Query

To get our list of volunteers from the database, we need a query that gathers the information we need (in this case, their name) from the records that we know are from volunteers. Referring to the method in the Multiple Registration Forms article, we have a field in the form that is filled with the name of the page the form was on. This is how we will know whether the record was created by a volunteer or not. Let’s say the volunteer’s form is on a page named “volunteer-signup” and the coordinator’s form is on a page called “volunteer-coordinators.” To get one and not the other we need two statements.

Here is our query:

SELECT first_name,last_name 
FROM wp_participants_database
WHERE form_type LIKE "%volunteer%" 
  AND form_type NOT LIKE "%coordinator%"

That will get us our list of names of all the volunteers.

With WordPress, it’s best to use the $wpdb object for interacting with the database. To get an array of values, we use the $wpdb->get_results method like this:

global $wpdb;
$values = $wpdb->get_col( '
   SELECT first_name,last_name 
   FROM `' . $wpdb->prefix . 'participants_database` 
   WHERE form_type LIKE "%volunteer%" 
     AND form_type NOT LIKE "%coordinator%"
' ); 

There is no user input here so we don’t need to sanitize the query. This will give us an array of objects, one for each record returned.

Altering the Dropdown

This part is pretty simple if you are familiar with using WordPress actions. When the action is triggered, our function will be called, and it is given the Participants Database form element object for the current form element. Our action, in this case is documented as: pdb-form_element_build_{$type} which means if we want to use it on a dropdown-type form element, the final action slug will be pdb-form_element_build_dropdown. The action is triggered on every dropdown form element, so we need to check to see if the current element is the one we want to add the options to. Because it is an action, we won’t be returning any value. As is often the case with actions, we can change the state of the given object in the function that is triggered by the action. When that object is used next (such as to print the form element) it will include our changes.

The Plugin

Here is the fully-commented code for the plugin:

That can be downloaded as a ZIP file here, then uploaded to your WordPress using the “Add Plugin” functionality.

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