Use AWS SDK DynamoDB Document Client to support nested scan

[lrcry]

Finally I've got one workaround for now to support nested scan via aws-sdk dynamodb document client...

To use it (as a promise), just compose your own filter ~~~and specify the option {useRawAwsFilter: true}~~~ when scanning:

    var Dog = dynamoose.model('Dog');
    // this should be composed, ref: http://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_Scan.html
    var filter = {
      FilterExpression: 'details.timeWakeUp = :wakeUp',
      ExpressionAttributeValues: {
        ':wakeUp': '8am'
      }
    };

    Dog.scan(filter).exec()
      .then(function(dogs) {
        console.log(dogs);
      })
      .catch(function(err) {
        console.error(err);
      });

It is not necessary to provide TableName in the filter as the library will use the model name as table name by default, if a TableName is not explicitly specified.
I didn't touch anything which used the default DynamoDB client. I think it would be a good idea to modify dynamoose to use the DocumentClient to support more features, although it means an amount of work. :)

Limitations:

• When defining schemas, please use { useDocumentTypes: true } if you would like to use nested scan as this is only supported for documents (I think it is a feature of DocumentClient, I googled that before but I could not remember the link, sorry for that)
• It seems that aws-sdk document client has not supported usage of attribute name placeholders yet (when I tried to use the plain aws-sdk in my code, it seemed never to work), which means that something as follows (#timeWakeUp) will get error:

{
  "FilterExpression": "#timeWakeUp = :wakeUp",
  "ExpressionAttributeNames": {
    "#timeWakeUp": "details.timeWakeUp"
  },
  "ExpressionAttributeValues": {
    ":wakeUp": "8am"
  }
}

Instead, use the name directly (details.timeWakeUp):

{
  "FilterExpression": "details.timeWakeUp = :wakeUp",
  "ExpressionAttributeValues": {
    ":wakeUp": "8am"
  }
}

[brandongoode]

@lrcry Thanks for the PR -- sorry it took so long for me to review the change.

[brandongoode]

I think it would be easier for other if we just check for FilterExpression in the filter.

if(type filter.FilterExpression === 'string')

[brandongoode]

I think this line should be removed

[lrcry]

Thanks. Let me check this to see what is going on at this point. I remembered I've already moved the AWS things into the model to keep consistency...

[brandongoode]

if(this.filter && type this.filter.FilterExpression === 'string')

[lrcry]

@brandongoode Can you please review my latest change as per the last review you've posted? Thanks a lot.

[brandongoode]

Looks great! Could you add a section in the Scan documentation and I'll merge it: https://github.com/automategreen/dynamoose/blob/master/docs/_docs/scan.md

[lrcry]

@brandongoode
I've done the document update. Please view it at #167