How Can I Dynamically Assign Field Values in MongoDB Using JavaScript?

Evaluating JavaScript in MongoDB for Dynamic Field Assignment

You seek to dynamically assign values to document fields based on JavaScript expressions, such as retrieving the current server timestamp. Your current approach, where you use a helper function such as mongoNow(), stores the JavaScript code as a script, preventing it from being evaluated.

The solution lies in MongoDB's server-side code execution capability. MongoDB stores reusable JavaScript functions in a system collection called system.js. By storing your code in system.js, you can later execute it.

However, note that server-side code execution in MongoDB has limited support and performance. For example, you cannot call stored procedures from within an insert statement.

To call a stored procedure from Go using the mgo driver, you can use the mgo.Database type's Run() method. This method allows you to issue an eval command with the JavaScript code to be executed server-side. Here's an example:

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

Alternatively, you can use the $eval operator in aggregation pipeline queries to dynamically generate field values based on JavaScript expressions. The $eval operator allows for more complex computations and transformations within the pipeline.

While server-side code execution provides flexibility, it is important to consider its limitations and potential performance implications. Ensure thorough testing and profiling before deploying code that relies on this functionality in production environments.

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