Go type assertion for MongoDB []bson.M{}/primitive.A type

[Tong Sun]

Further on with the question 

https://stackoverflow.com/questions/60072372/filtering-values-out-of-a-bson-m-in-golang/

where the answer says to use `range server.installed` while the comment says

server.host undefined (type string has no field or method subject)

This is what I've tried with the MongoDB `[]bson.M{}` type results (obtained from like `$lookup`):

The []bson.M{} type of result I'm getting is:

[map[_id:ObjectID("65529d178688bac31b739f82") author:[map[_id:ObjectID("65529d168688bac31b739f81") created_at:1699912982379 name:Mehran updated_at:1699912982379]] author_id:ObjectID("65529d168688bac31b739f81") created_at:1699912983386 name:Test pages:124 updated_at:1699912983386]]

I've made several attempts but was unable to access the author field of the lookup result in Go.

a := []author{} 

for _, val := range result { a = append(a, val["author"].(primitive.A)[0]) }

• If using val.author, I'm getting val.author undefined (type primitive.M has no field or method author)
• If using val["author"], I'm getting invalid operation: cannot index val["author"] (map index expression of type interface{})
• If using val["author"].(primitive.A)[0], I'm getting cannot use val["author"].(primitive.A)[0] (variable of type interface{}) as author value in argument to append: need type assertion

And how to go about to fix this "need type assertion" just stumbled me, as I've tried val["author"].(primitive.A).([]main.author)[0], or []author(val["author"].(primitive.A))[0], but I'm getting invalid operation: cannot call non-function val["author"].(primitive.A) (comma, ok expression of type primitive.A)

and I've now run out of ideas how to fix it. 

Please help

[burak serdar]

val["author"].(primitive.A)[0] is correct, but the type of that
expression is not author, it is bson.M. You might consider using a
tagged struct to unmarshal this.

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[Tong Sun]

Thanks for the hint. would you elaborate a bit more please as I'm still unsure how to do it. and the closes hits that I found are

• https://stackoverflow.com/questions/71739971/unmarshalling-a-bson-d-object-golang
• https://www.mongodb.com/docs/drivers/go/current/fundamentals/bson/
  

But neither covers how to unmarshal bson.M types. I got either:

cannot use r (variable of type interface{}) as []byte value in argument to bson.Unmarshal: need type assertion

Or with:

r := val["author"].(primitive.A)[0]
var a1 author
bson.Unmarshal(r.([]byte), &a1)

a = append(a, a1)

I'm getting:

panic: interface conversion: interface {} is primitive.M, not []uint8

On Thursday, November 16, 2023 at 12:20:13 PM UTC-5 burak serdar wrote:

val["author"].(primitive.A)[0] is correct, but the type of that
expression is not author, it is bson.M. You might consider using a
tagged struct to unmarshal this.

[burak serdar]

Something like this:

type Author struct {
ID primitive.ObjectID `bson:"_id"`
CreatedAt time.Time `bson:"created_at"`
...
}

type Data struct {
Author []Author `bson:"author"`
}


Then you read the results into var result []Data

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