How to Retrieve MongoDB Documents by an Array of Object IDs using mgo and bson?

Retrieving MongoDB Documents by Array of Object IDs

In MongoDB, it's possible to store an array of object IDs (also known as _ids) within a document. To retrieve all documents corresponding to those object IDs efficiently, use the $in operator.

Question:

You have an array of hex-encoded _ids:

ids := ["543d171c5b2c12420dd016", "543d171c5b2dd016"]

How do you retrieve the corresponding documents using mgo and bson?

Answer:

Your initial approach using:

query := bson.M{"_id": bson.M{"$in": ids}}
c.Find(query).All()

is incorrect because the _ids are stored as object IDs, not strings. To retrieve documents by object IDs, you need to convert the hex-encoded strings into bson.ObjectId objects:

oids := make([]bson.ObjectId, len(ids))
for i := range ids {
  oids[i] = bson.ObjectIdHex(ids[i])
}

query := bson.M{"_id": bson.M{"$in": oids}}
c.Find(query).All()

This code:

1. Creates a slice of bson.ObjectId objects.
2. Iterates through the hex-encoded _id strings, converting each to an object ID using the bson.ObjectIdHex function.
3. Constructs a query where the _id field matches any of the provided object IDs.
4. Uses the Find and All methods to retrieve all matching documents from the MongoDB collection.

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