Example plugin for Lift Search. The below example will add a comment count field to the search handing. Another example for adding a taxonomy can be see within the plugin, https://github.com/voceconnect/lift-search/blob/master/wp/field.php#L711.

lift-comment-facet.php

<?php

define( 'lift_comment_facet_version', 1 );
add_action( 'init', function() {
		if ( class_exists( 'Lift_Search' ) ) {
										//field name, field type
			$comment_field = liftField( 'comment_count', 'uint' )
				//add field to parse_request handling so it gets passed to the global WP_Query
				->addRequestVars( 'comment_count' )
				//set the delegate for adding the field value to the document sent to CS
				->delegate( 'getDocumentValue', function($post_id) {
						return intval( get_comments_number( $post_id ) );
					} )
				//set the delegate that converts request vars to WP_Query query_vars
				->delegate( 'requestToWP', function($request) {
						if ( isset( $request['comment_count'] ) ) {
							if ( false === strpos( $request['comment_count'], '..' ) ) {
								$request['min_comments'] = $request['max_comments'] = abs( intval( $request['comment_count'] ) );
							} else {
								$count_parts = explode( '..', $request['comment_count'] );

								if ( $count_parts[0] )
									$request['min_comments'] = $count_parts[0];

								if ( $count_parts[1] )
									$request['max_comments'] = $count_parts[1];
							}
							unset( $request['comment_count'] );
						}
						return $request;
					} )
				//set the delegate that converts WP_Query query_vars to a boolean query
				->delegate( 'wpToBooleanQueryParam', function($lift_query) {
						$wp_query = $lift_query->wp_query;
						$min_comments = intval( $wp_query->get( 'min_comments' ) ) ? $wp_query->get( 'min_comments' ) : '';
						$max_comments = intval( $wp_query->get( 'max_comments' ) ) ? $wp_query->get( 'max_comments' ) : '';

						if ( $min_comments || $max_comments ) {
							if ( $min_comments == $max_comments ) {
								return "comment_count:$min_comments";
							} else {
								return "comment_count:$min_comments..$max_comments";
							}
						}
						return null;
					} )
				->delegate( 'setFacetOptions', function($cs_query, $field ) {
						$cs_query->add_facet_contraint( $field->name, array(
							'..5', '6..10', '11..20', '21..'
						) );
					} )
				->delegate( 'wpToLabel', function($query_vars) {
					$min = isset($query_vars['min_comments'])  ? intval($query_vars['min_comments']) : '';
					$max = isset($query_vars['max_comments'])  ? intval($query_vars['max_comments']) : '';
					if($min && $max) {
						return sprintf('%d to %d', $min, $max);
					} elseif($min) {
						return sprintf("More than %d", $min);
					} elseif($max) {
						return sprintf("Less than %d", $max);
					} else {
						return "Any";
					}
				} );

			$range_filter = new LiftSelectableFacetControl( $comment_field, 'Comment Count' );

			//enqueue comment changes to mark documents as changed
			add_action( 'wp_update_comment_count', function($post_id) {
					Lift_Document_Update_Queue::queue_field_update( $post_id, 'comment_count' );
				} );

			//check if the version of this plugin changed and update the schema if needed.
			$installed_version = get_option( 'lift_comment_facet_version', 0 );
			if ( $installed_version < 1 ) {
				Lift_Search::update_schema();
			}
		}
	}, 9 );

Hi Mike,

I can't find a method "liftField" anywhere in the project. Was this something you defined?