Redis social graph in node.js, with following and blocking

graph.js

var redis = require('redis').createClient(),
    f = require('util').format;

Array.prototype.diff = function(a) { // use to remove blocked users
    return this.filter(function(i) {return !(a.indexOf(i) > -1);});
};

var methods = {
  follow: function(a, b, cb) {
   db.sadd(f("node:%s:following", a), b, function(err, res){
      db.sadd(f("node:%s:followers", b), a, function(e, re) {
        if(err || e) {
          return cb(err || e);
        } else {
          return cb(null);
        }
      });
   });
  },
  unfollow: function(a, b, cb) {
   db.srem(f("node:%s:following", a), b, function(err, res){
      db.srem(f("node:%s:followers", b), a, function(e, re) {
        if(err || e) {
          return cb(err || e);
        } else {
          return cb(null);
        }
      });
   });
  },
  block: function(a, b, cb) {
    db.sadd(f("node:%s:blocked", a), b, function(err, res) {
      db.saad(f("node:%s:blockers", b), a, function(e, re) {
        if(err || e) {
          return cb(err || e);
        } else {
          return cb(null);
        }
      });
    });
  },
  unblock: function(a, b, cb) {
    db.srem(f("node:%s:blocked", a), b, function(err, res) {
      db.srem(f("node:%s:blockers", b), a, function(e, re) {
        if(err || e) {
          return cb(err || e);
        } else {
          return cb(null);
        }
      });
    });
  },
  following: function(a,l,cb,u) {
    db.sort([f("node:%s:following", a), "ALPHA", "LIMIT", l.offset || l.o || l.off || 0, l.limit || l.l || l.lim || 25], function(err, res) {
      if(u) {
        this.blocks(u, {o:0,l:-1}, function(e,r) {
          return cb(err, res.diff(r))
        });
      } else {
        return cb(err, res);
      }
    });
  },
  followers: function(a,l,cb,u) {
    db.sort([f("node:%s:followers", a), "ALPHA", "LIMIT", l.offset || l.o || l.off || 0, l.limit || l.l || l.lim || 25], function(err, res) {
      //console.log([f("node:%s:followers", a), "ALPHA", limitStr])
      if(u) {
        this.blocks(u, {o:0,l:-1}, function(e,r) {
          return cb(err, res.diff(r))
        });
      } else {
        return cb(err, res);
      }
    });
  },
  blocks: function(a,l,cb,u) {
    db.sort([f("node:%s:blocks", a), "ALPHA", "LIMIT", l.offset || l.o || l.off || 0, l.limit || l.l || l.lim || 25], function(err, res) {
      return cb(err, res);
    });
  },
  blockers: function(a,l,cb,u) {
    db.sort([f("node:%s:blockers", a), "ALPHA", "LIMIT", l.offset || l.o || l.off || 0, l.limit || l.l || l.lim || 25], function(err, res) {
      return cb(err, res);
    });
  },
  friends: function(a,cb,u) {
    db.sinter([f("node:%s:following", a), f("node:%s:followers", a)], function(err, res) {
      if(u) {
        this.blocks(u, {o:0,l:-1}, function(e,r) {
          return cb(err, res.diff(r))
        });
      } else {
        return cb(err, res);
      }
    });
  },
  followingCount: function(a,cb) {
    db.scard(f("node:%s:following", a), function(err, res) {
      return cb(err, res);
    });
  },
  followerCount: function(a,cb) {
    db.scard(f("node:%s:followers", a), function(err, res) {
      return cb(err, res);
    });
  },
  friendsCount: function(a,cb) {
    this.friends(a, function(err, r) {
      return cb(err, r.length);
    });
  },
  isFollowing: function(a,b,cb) {
    db.sismember([f("node:%s:following", a), b], function(err, res) {
      return cb(err, res);
    });
  }
};

module.exports = methods;