$concatArrays only supports arrays, not object

Question

I'm trying to code a perfectly working MongoDB query in Go, but I'm having a hard time with arrays.

Working on JSON:

[
...
{
    $project: {
        acl: {
            $reduce: {
                input: "$a.accesses",
                initialValue: [],
                in: {
                    $concatArrays: ["$$value", "$$this"]
                }
            }
        }
    }
}]

But not working on Go:

pipe := mongo.Pipeline{
    ...
    bson.D{{Key: "$project", Value: bson.M{
        "acl": bson.M{
            "$reduce": bson.M{
                "input":        "$a.accesses",
                "initialValue": bson.M{},
                // None of the below works
                "in": bson.M{"$concatArrays": bson.A{"$$value", "$$this"}},
                // "in": bson.M{"$concatArrays": []interface{}{"$$value", "$$this"}},
                // "in": bson.M{"$concatArrays": [2]string{"$$value", "$$this"}},
                // "in": bson.M{"$concatArrays": []string{"$$value", "$$this"}},
                // "in": bson.M{"$concatArrays": []interface{}{"$$value", "$$this"}},
                // "in": bson.D{{Key: "$concatArrays", Value: []interface{}{"$$value", "$$this"}}},
            },
        },
    }}},
}

Error: $concatArrays only supports arrays, not object

I'm new in Go so I'm quite sure I'm missing the concept of arrays somewhere.

Answer 1

The Go value you supply for initialValue is not an array:

"initialValue": bson.M{},

Instead do:

"initialValue": []interface{}{},

Or:

"initialValue": bson.A{},