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<?php
/**
 * Gravity Wiz // Gravity Forms // Populate Quantity Max Range from Custom Field
 * http://gravitywiz.com
 */
add_filter( 'gform_pre_render', function( $form ) {

	foreach( $form['fields'] as &$field ) {
		if( $field['id'] == 1 ) {
			$field['rangeMax'] = get_post_meta( get_queried_object_id(), 'my_custom_field', true );
		}
	}

	return $form;
} );