Golang - Appengine datastore filter query with []byte comparison

Question

I am trying to perform a filter query on a set of entities in the datastore, but the entity field I am trying to query against, with the equality operator, is of type []byte and I don't know if appengine datastore can perform this comparison

This is my entity:

type Data struct {
 Id          int64  `json:"id"`
 Version     int32  `json:"-"`
 HMAC        []byte `json:"-"`
 Status      string `json:"status"`
}

And here is my query logic

func (view *DataView) GetDataByHMAC(hmac []byte) (Data, error) {
    view_key := datastore.NewKey(view.context, "View", "data-view", 0, nil)
    data := make([]Data, 0)
    query := datastore.
       NewQuery("ViewData").
       Ancestor(view_key).
       Filter("HMAC = ", hmac)
    _, err := query.GetAll(view.context, &data)
    if err != nil {
       return Data{}, err
    }
    if len(data) == 0 {
       return Data{}, ErrNoData
    }
    return data[0], nil
}

It builds but does not return any results, even after programmatically retrying over the course of 10 seconds so i do not believe it is an issue of eventual consistency between the datastore and the view data that I've stored there.

My main question is: does the appengine datastore allow for a query to use a comparison filter on a field with type []byte?

Before I spend any time on this remove the trailing space in the Filter string "HMAC = " and see what happens. — AndrewN
I see no change in behavior and all of my other queries include both optional spaces and return results accurately; I hadn't thought of checking though, thanks — InternetProfessionalPersonHere

djd djd · Accepted Answer · 2014-09-16T02:48:08

In 1.9.11, the ByteString type was introduced to the datastore package. It can be used for storing short, indexed byte slices.

If you change your entity to the following, it should work:

type Data struct {
  ID      int64                `json:"id"`
  Version int32                `json:"-"`
  HMAC    datastore.ByteString `json:"-"`
  Status  string               `json:"status"`
}

More info: https://developers.google.com/appengine/docs/go/datastore/entities#Go_Properties_and_value_types

Golang - Appengine datastore filter query with []byte comparison

2 Answers