code: function postSetTags(post_id, tags, cb) {
    var ids = _.pluck(tags, 'id')
      , q1 = 'DELETE FROM posts_tags WHERE post_id = $1'
      , q2 = 'INSERT INTO posts_tags (post_id, tag_id) VALUES ' +
      ids.map(function(id) {
        return '(' + post_id + ',' + id + ')';
      }).join(', ');
  
    db.getClient(function(err, client, done) {
      client.query(q1, [post_id], function(err, r) {
        if(err) { return cb(err); }
        client.query(q2, function(err, r) {
          if(err) {
            cb(err);
            done(err);
          } else {
            cb(null, r.rowCount);
            done();
          }
        });
      });
    });
  }
docs: Removes existing association and associates `tags` with post with id `post_id`
  
  @param {Number} post_id
  @param {Array} tags
  @param {Function} cb