toString() {
    return `[${this.type}/${this.id}: ${
      this.from && this.to
        ? `${this.from.toString()} => ${
            this.to.isAsset
              ? this.to.toString()
              : this.to.url || this.to.type || '?'
          }`
        : 'unattached'
    }]`;
  }
Get a brief text containing the type, id of the relation. Will
also contain the `.toString()` of the relation's source and
target assets if available.

@return {String} The string, eg. `"[HtmlAnchor/141: [Html/40 file:///foo/bar/index.html] => [Html/76 file:///foo/bar/otherpage.html]]"``