WordPress Advanced Custom Fields get field key from field name

acf-get-field-key.php

<?php

/**
 * Get field key for field name.
 * Will return first matched acf field key for a give field name.
 * 
 * ACF somehow requires a field key, where a sane developer would prefer a human readable field name.
 * http://www.advancedcustomfields.com/resources/update_field/#field_key-vs%20field_name
 * 
 * This function will return the field_key of a certain field.
 * 
 * @param $field_name String ACF Field name
 * @param $post_id int The post id to check.
 * @return 
 */
function acf_get_field_key( $field_name, $post_id ) {
	global $wpdb;
	$acf_fields = $wpdb->get_results( $wpdb->prepare( "SELECT ID,post_parent,post_name FROM $wpdb->posts WHERE post_excerpt=%s AND post_type=%s" , $field_name , 'acf-field' ) );
	// get all fields with that name.
	switch ( count( $acf_fields ) ) {
		case 0: // no such field
			return false;
		case 1: // just one result. 
			return $acf_fields[0]->post_name;
	}
	// result is ambiguous
	// get IDs of all field groups for this post
	$field_groups_ids = array();
	$field_groups = acf_get_field_groups( array(
		'post_id' => $post_id,
	) );
	foreach ( $field_groups as $field_group )
		$field_groups_ids[] = $field_group['ID'];
	
	// Check if field is part of one of the field groups
	// Return the first one.
	foreach ( $acf_fields as $acf_field ) {
		if ( in_array($acf_field->post_parent,$field_groups_ids) )
			return $acf_field->post_name;
	}
	return false;
}

Awesome, this did the job to me. Seems strange ACF doesn't offer an easier way to discover the field keys given that it is necessary for lots of useful things. For example I want to let users edit and create custom post types on the front end, but I don't want to give them access to all the fields. In order to do that I need the field keys. It was most vexing trying to find them!

Thank you so much! I looked everywhere for how to do this. Context was how to get a select's options from within a repeater, for every post independent of a particular post ID.

This looks like just what I need, but embarrassingly, I don't know what to do with this file. Do I add the entire PHP file somewhere or just take the function into functions.php?

@bodiequirk copy+paste function into your theme's functions.php file, or you can save the file and require_once("/path/to/acf-get-field-key.php");.

Thanks for sharing! It helped a lot. Looks like there is a bug on line 39. Should be
return $acf_field->post_name;

dc2ed-5c539
9e224-1292e
ed9d8-c0707
5fbf5-8a14d
736f6-fce98
56ca0-ba9d7
8b149-cb6e6
c67f9-81158
58636-5a95c
259c9-de3bc
a607f-1a051
9b332-6da2d
68584-daf19
f20f2-024d4
2cb9e-a7bea
a505f-6971b

Smart! I was looking for such solution.
BUT, realizing that I shall generate zillion redundant accesses to the database
I shall simply build a map table, upon plugin activation.
Something like t(field-group, sub-field) = key-value
Using your code.
So I do it once and app user will not notice...
Thanks!!!

This code will blow up your database

Thank you!

If you need to use this code outside your theme and don't have access to ACF functions (like in another WordPress plugin) you can include a line similar to below to reference the ACF plugin (like ACF Pro plugin):

include_once WP_PLUGIN_DIR .'/advanced-custom-fields-pro/acf.php';

This code will blow up your database

@ShinekhuuD can you please tell us an alternative if you don't think this is a good idea?

@Hollyw00d Allow me. There are few ways:

1. Use get field object - if you have the exact selector, its simple
2. Parse the acf-json/*.json and extract keys from there. It can done on the fly Or you may generate your own index
   Hope that helps

@Mulli I used your suggestion below to add the ACF field ID (like field_63c84b9cb5f66):

Parse the acf-json/*.json and extract keys from there. It can done on the fly Or you may generate your own index

I can't display the code here as I used it for my job but this another option to get the ACF field ID without making database calls.

Or one can simply use acf_get_reference()