async addMetadata (bookmarkId, metadataId) {
    const num = await db('bookmarks_mapping').where('bookmarkId', bookmarkId).where('metadataId', metadataId).count()

    if (num && num[0]['count(*)'] !== 0) return true

    let result = await db('bookmarks_mapping').insert({
      bookmarkId,
      metadataId,
      updateTime: (new Date()).getTime()
    }).select('id')

    result = result[0]

    return result
  }
Add metadata to bookmark

@param {Int} bookmarkId bookmark id
@param {Int} metadataId matedata id

@returns {Int} bookmark mapping id