How to Efficiently Retrieve MongoDB Documents by ID Using mgo's FindId() Method?

Accessing Data by ID Using Mgo

In Mongo database, finding a specific document by its unique identifier, known as the "_id" field, is a common task. Here's a comprehensive guide on how to accomplish this using mgo, a popular MongoDB driver for Go.

Understanding mgo's FindId() Method

The FindId() method is specifically designed to fetch a document based on its "_id" field. It takes an argument of type bson.ObjectId or a string representing an ObjectId. However, it's important to note that when using the FindId() method, you should only provide the ObjectId value, not the entire bson.M{} map as an argument.

Correct Usage of FindId() and Find()

In your code, the issue lies in the incorrect usage of the FindId() method. Here's the corrected code that should work:

err2 := c.FindId(bson.ObjectIdHex("58593d1d6aace357b32bb3a1")).One(&data)

Alternatively, you can use the Find() method along with specifying the "_id" field and value in the bson.M{} map:

err2 := c.Find(bson.M{"_id": bson.ObjectIdHex("58593d1d6aace357b32bb3a1")}).
    One(&data)

Verifying Data Retrieval

If the document is found, you will not encounter any errors during the execution of the code. However, if the code still returns a 0 value for data.IdCookie, it suggests that the field name for the id in the database is different from the one you've specified in your struct.

Using Struct Tags for Mapping

To ensure proper mapping between your Go struct and the MongoDB document, you should use struct tags. For instance, if the "_id" field in the database is named "myid," you can map it to your struct as follows:

type id_cookie struct {
    IdCookie int `bson:"myid"`
}

Optimizing Connection Handling

To enhance performance, you should consider avoiding multiple connections to the MongoDB server for each query. Establishing a single session and reusing it for multiple queries can significantly speed up operations. For more information on optimizing connection handling, refer to: [mgo - query performance seems consistently slow (500-650ms)](https://github.com/mongodb/mongo-go-driver/issues/1028)

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