How to Effectively Filter MongoDB Fields Using the mongo-go-driver?

Filtering Fields in MongoDB Documents with the mongo-go-driver

One of the key capabilities ofMongoDB queries is the ability to filter the fields returned in the query results. This is known as field projection.

Querying with Field Projection

The mongo-go-driver provides several options for specifying field projections. One option is to use the findopt.Projection function. However, as you have encountered, this approach can fail if the field names in the projection struct are not exported. To address this issue, you can use the following strategies:

Method 1: Use Exported Field Names

The most straightforward approach is to use exported field names (i.e., fields that start with uppercase letters) in your projection struct. This ensures that the field names are accessible to the mongo-go-driver.

type Fields struct {
    ID int `bson:"_id"`
}

projection := Fields{
    ID: 0,
}

Method 2: Utilize Struct Tags

Alternatively, you can use struct tags to map unexported field names to the corresponding MongoDB field names.

type Fields struct {
    _id int `bson:"_id"`
}

projection := Fields{
    _id: 0,
}

Using the Projection in the Query

Once you have defined the projection, you can use it in the FindOne or Find methods by setting the SetProjection option.

filter := bson.NewDocument(bson.EC.ObjectID("_id", starterId))
result := staCon.collection.FindOne(nil, filter, options.FindOne().SetProjection(projection)).Decode(s)

Example Using bson.M

As of a later version of the mongo-go-driver, you can also specify the projection using a simple bson.M map.

options.FindOne().SetProjection(bson.M{"_id": 0})

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