Source

File: wp-includes/rest-api/endpoints/class-wp-rest-controller.php

public function get_fields_for_response( $request ) {
		$schema     = $this->get_item_schema();
		$properties = isset( $schema['properties'] ) ? $schema['properties'] : array();

		$additional_fields = $this->get_additional_fields();

		foreach ( $additional_fields as $field_name => $field_options ) {
			// For back-compat, include any field with an empty schema
			// because it won't be present in $this->get_item_schema().
			if ( is_null( $field_options['schema'] ) ) {
				$properties[ $field_name ] = $field_options;
			}
		}

		// Exclude fields that specify a different context than the request context.
		$context = $request['context'];
		if ( $context ) {
			foreach ( $properties as $name => $options ) {
				if ( ! empty( $options['context'] ) && ! in_array( $context, $options['context'], true ) ) {
					unset( $properties[ $name ] );
				}
			}
		}

		$fields = array_keys( $properties );

		if ( ! isset( $request['_fields'] ) ) {
			return $fields;
		}
		$requested_fields = wp_parse_list( $request['_fields'] );
		if ( 0 === count( $requested_fields ) ) {
			return $fields;
		}
		// Trim off outside whitespace from the comma delimited list.
		$requested_fields = array_map( 'trim', $requested_fields );
		// Always persist 'id', because it can be needed for add_additional_fields_to_object().
		if ( in_array( 'id', $fields, true ) ) {
			$requested_fields[] = 'id';
		}
		// Return the list of all requested fields which appear in the schema.
		return array_reduce(
			$requested_fields,
			function( $response_fields, $field ) use ( $fields ) {
				if ( in_array( $field, $fields, true ) ) {
					$response_fields[] = $field;
					return $response_fields;
				}
				// Check for nested fields if $field is not a direct match.
				$nested_fields = explode( '.', $field );
				// A nested field is included so long as its top-level property
				// is present in the schema.
				if ( in_array( $nested_fields[0], $fields, true ) ) {
					$response_fields[] = $field;
				}
				return $response_fields;
			},
			array()
		);
	}