How Can I Evaluate JavaScript Expressions to Dynamically Assign Field Values in MongoDB?

Evaluating JavaScript Expressions in MongoDB Field Values

In MongoDB, it's possible to dynamically assign field values based on the evaluation of JavaScript expressions. However, simply inserting a JavaScript object as a field value won't result in the expression being evaluated.

Server-Side Code Execution

MongoDB provides a mechanism for storing and executing JavaScript functions on the server side. The special collection system.js stores these functions. To access this collection and execute a function, you can use the Run() method of the mgo.Database type. The Run() method takes an eval command with the JavaScript code to be executed as an argument.

For example, to call a stored function named myStoredFunction() from Go using the mgo driver, you can use the following code:

db.Run(bson.M{"eval": "myStoredFunction();"})

Example

Suppose you want to set the lastSeen field to the current server time. You can create a stored JavaScript function in system.js called getCurrentTime():

function getCurrentTime() {
  return (new Date()).toISOString();
}

Then, in your insert statement, you can reference the stored function as follows:

err := c.Insert(
   struct{Serial, Priority, Url, LastSeen interface{}}{ 
      Url: getInformedHost() + ":" + getRunningPortString(), 
      Priority: rand.Int(), 
      LastSeen: bson.M{"$eval": "getCurrentTime()"}
   }
)

By using $eval in the LastSeen field value, MongoDB will evaluate the specified JavaScript expression and store the result as the field value.

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