dynamodb: can't run a "CountQuery" with an index

[jonathaningram]

This is because there is no such method. You can't use QueryTable because it requires the JSON response to have an Items key (a dynamodb count call doesn't seem to return one) E.g. if you try some code like this:

q := dynamodb.NewQuery(table)
q.AddIndex("myIndex")
  q.AddKeyConditions([]dynamodb.AttributeComparison{
    *dynamodb.NewEqualStringAttributeComparison(AttributeStatus, status),
  })
q.AddSelect("COUNT")
res, _, err := t.QueryTable(q)
log.Println(err)

You'll get an error like this:

Unexpected response {"Count":207,"LastEvaluatedKey":{"Id":{"S":"a2f0983e-9e2a-4d67-ac88-8fde867cc250"},"Status":{"S":"Complete"}},"ScannedCount":207}

(i.e. no Items key exists)

I'd like to add a new method similar to https://github.com/crowdmob/goamz/blob/master/dynamodb/query.go#L38

func (t *Table) CountQueryOnIndex(attributeComparisons []AttributeComparison, indexName string) (int64, error) {
  // ...
}

Also, I'll move the bulk of the CountQuery method into a helper func to reduce duplication.

Would you accept a PR with that?

[alimoeeny]

Again, we appreciate your contribution. Please send your PR. BUT, please take this into account. Unless there is no way around it, at the moment we don't want to make backward incompatible changes. We are working on a solution that let's us move forward and refactor the libraries and clean up the code in a backward incompatible way without breaking people's code. but until then, please define new functions with new signatures as needed.

[jonathaningram]

@alimoeeny great thanks I'll work on a PR. Yep it's a new signature, everything else stays the same.