Why Are My Golang mgo Queries Returning Empty Objects?

How to Avoid Empty Objects with Golang's Mgo

When using Golang's mgo package to interact with MongoDB, it's crucial to understand the proper usage of BSON and JSON tags. Without using appropriate tag annotations, data retrieval may result in empty objects.

The code snippet provided attempts to query MongoDB for records with a specific "user" field. However, the returned records are empty due to a common oversight: the struct fields are not exported. To resolve this issue, modify the users struct to export the User and Data fields:

type users struct {
    User string `bson:"user" json:"user"`
    Data string `bson:"data" json:"data"`
}

By exporting the struct fields, you instruct the mgo package to use these field names during data transformation and retrieval. Without this export, the package will ignore the fields.

Additionally, it's essential to understand the role of BSON and JSON tags. In MongoDB, data is stored in BSON (Binary JSON) format. However, Go uses JSON to represent data structures. The BSON and JSON tags allow you to specify how Go fields map to their corresponding BSON counterparts.

In the provided code, the User and Data tags specify that the Go struct fields should be mapped to the "user" and "data" fields in BSON, respectively. By using these tags, you ensure that data is stored and retrieved in the correct format.

By following these guidelines, you can effectively avoid retrieving empty objects and ensure the proper handling of data between Go and MongoDB using the mgo package.

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