How Can I Use Stored JavaScript Functions in MongoDB to Dynamically Assign Field Values?

Evaluating JavaScript in MongoDB to Assign Field Values

Inserting documents into a MongoDB collection can be augmented with JavaScript evaluation to dynamically assign field values. However, the code given initially fails to evaluate the JavaScript statement and instead stores it as a script.

The MongoDB documentation outlines a method for storing JavaScript functions server-side in the system.js collection. By creating a stored function, we can evaluate JavaScript remotely from the Go client.

Method using Stored Functions:

1. Create a function in the system.js collection:

db.system.js.insert({
    _id: "assignDate",
    value: function() {
        return ISODate();
    }
});

2. Call the stored function from Go using the Run() method:

func createInstance(c *mgo.Collection) {
    result, err := c.Run(
        bson.M{"eval": "assignDate()"},
    )
    checkError(err, "Could not get server time")

    var doc bson.M
    err = result.One(&doc)
    if err != nil {
        checkError(err, "Invalid result from stored function", 3)
    }
    lastSeen, ok := doc["retval"].(time.Time)
    if !ok {
        checkError(fmtErrorf("Invalid result from stored function: %v", doc["retval"]), "Invalid result from stored function: %v", 3)
    }

    // ... Insert document with evaluated field value
}

Considerations:

• Server-side code evaluation in MongoDB still has limitations.
• Using stored functions provides a workaround for evaluating JavaScript, but it's important to note its performance caveats.

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