gistfile1.js

// a function to make sure you're always updating
// the newest version of a document.
//
// It will fight to apply your `operation`.

function updateDocument(
  id, // id of the document to update
  operation, // function that will alter the document
  callback // called when it's done
){
  var that_function = this;
  var those_arguments = arguments;
  
  db.get(id, function(error_getting_doc, doc){
    if(error_getting_doc){
      return callback(error_getting_doc, null)
    }
    
    // enclose this in a `try` statement so that you can 
    // throw an error inside your operation if you need to
    try{
      var updated_doc = operation(doc);
    } catch(update_error){
      return callback(error, null)
    }
    
    // your update operation needs to return the document 
    // you want to insert
    if (updated_doc == undefined) return callback({
      error: "operation_error",
      message: "operation returned undefined"
    })
    
    // here we try to insert the updated document
    db.insert(updated_doc, function(error, result){
      if(error){
        if(error.error == 'conflict'){
          
          // if there is a conflict error, call the 
          // uppermost function again with the same
          // arguments
          return that_function.apply(those_arguments)
        }
        
        // if the error wasn't caused by a conflict,
        // return it in the callback
        return callback(error, null)
      }
      
      // if there was no error, your document has
      // been successfully updated
      return callback(null, result)
    })
  })
}


// Example
updateDocument(
  '97969a83-5168-df82-a9bc-eb9f2c043dff',
  function(doc){
    doc.updated = (new Date()).getTime();
    return doc
  },
  function(error, callback){
    if(error){
      return console.log('there was a real error')
    }
    
    return console.log('document updated')
  }
)