Final MongoDB fixed-sized array example.

gistfile1.js

db.searches.remove()
db.searches.save(
  {_id: 1, 
   "searches-size" : 3, 
   "searches" : [
     {"site" : "google", "q": "restaurants"}, 
     {"site" : "yahoo", "q" : "shoes"}, 
     {"site" : "bing", "q" : "cheap flights"}
    ]
  }
)
db.searches.update(
  { _id : 1 }, 
  { $inc: { "searches-size" : 1 }, 
    $push : { "searches" : { "site" : "blekko.com", "q" : "mongodb" } } 
  }
);
db.searches.update(
  { _id : 1, "searches-size" : { $gt: MAX-ARRAY-LENGTH } }, 
  { $inc: { "searches-size" : -1 }, $pop : { "searches" : -1 } }
)