GAE Go — How to use GetMulti with non-existent entity keys?

Question

I've found myself needing to do a GetMulti operation with an array of keys for which some entities exist, but some do not.

My current code, below, returns an error (datastore: no such entity).

err := datastore.GetMulti(c, keys, infos)

So how can I do this? I'd use a "get or insert" method, but there isn't one.

Bludge

Answer 1

GetMulti can return a appengine.MultiError in this case. Loop through that and look for datastore.ErrNoSuchEntity. For example:

if err := datastore.GetMulti(c, keys, dst); err != nil {
    if me, ok := err.(appengine.MultiError); ok {
        for i, merr := range me {
            if merr == datastore.ErrNoSuchEntity {
                // keys[i] is missing
            }
        }
    } else {
        return err
    }
}

Deviationism

you could shorten that block if you used a type assertion. if me, ok := err.(appengine.MultiError); ok { for { ... } } – Octopod

Thanks. Is that what you meant? – Deviationism

yep you can drop the else clause as well and just return err for one less line too :-) – Octopod

In the else case there's a real error and we want to indicate some problem, so I think it makes sense to keep it. – Deviationism

Ahhh yeah since you still need access to the err object. Good point. – Octopod

Answer 2

I know this topic is up for more than a few days, but I like to post an alternative, using type switch.

if err := datastore.GetMulti(c, keys, dst); err != nil {
  switch errt := err.(type) {
  case appengine.MultiError:
    for ix, e := range errt {
      if e == datastore.ErrNoSuchEntity {
        // keys[ix] not found
      } else if e != nil {
        // keys[ix] have error "e"
      }
    }
  default:
    // datastore returned an error that is not a multi-error
  }
}

Policeman

Answer 3

Thought I'd throw my answer in to display another usecase. The following will take in any number of keys and return all the valid keys only.

// Validate keys
var validKeys []*ds.Key
if err := c.DB.GetMulti(ctx, tempKeys, dst); err != nil {
    if me, ok := err.(ds.MultiError); ok {
        for i, merr := range me {
            if merr == ds.ErrNoSuchEntity {
                continue
            }
            validKeys = append(validKeys, tempKeys[i])
        }
    } else {
        return "", err
    }
} else {
    // All tempKeys are valid
    validKeys = append(validKeys, tempKeys...)
}

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