如何在Swift中使用filterExpression执行DynamoDB扫描

Question

我有一个表，将有关组的信息（GroupID，成员，创建者，LastAccessed，GroupName等）存储为单独的行。 每个组都有一个唯一的标识符（GroupID）作为其哈希值（主键）。 它们还具有称为GroupName的属性。 我有一个搜索框，用户在其中输入部分组名称。 我想在表上执行扫描，并返回以用户输入开头的所有组。 这是我到目前为止所拥有的..

func searchForGroupsWithName(groupName: String) {

    self.queryInProgress = true

    let cond = AWSDynamoDBCondition()
    let v1    = AWSDynamoDBAttributeValue();
    v1.S = groupName

    cond.comparisonOperator = AWSDynamoDBComparisonOperator.BeginsWith
    cond.attributeValueList = [ v1 ]

    let exp = AWSDynamoDBScanExpression()

    //I only want to return the GroupName and GroupID.
    //I think this should be ["GroupID", "GroupName"], but it requires a string
    exp.projectionExpression = ??????????

    //I am not sure how to incorporate cond with this.
    exp.filterExpression = ??????????

    dynamoDBObjectMapper.scan(GroupTableRow.self, expression: exp).continueWithBlock({ (task:AWSTask!) -> AnyObject! in

        if task.result != nil {
            let paginatedOutput = task.result as! AWSDynamoDBPaginatedOutput

            for item in paginatedOutput.items as! [GroupTableRow] {
                self.searchBarResults.append(item)
            }

            if ((task.error) != nil) {
                print("Error: \(task.error)")
            }
            self.queryInProgress = false
            return nil
        }
        self.queryInProgress = false
        return nil
    })
}

Answer 1

在从WestonE获得帮助之后，我修正了我的代码并提出了一个可行的示例。 我在下面将其粘贴给其他需要类似内容的人

func searchForGroupsWithName(groupName: String) {

    self.queryInProgress = true

    let lowercaseGroupName = groupName.lowercaseString

    let scanExpression = AWSDynamoDBScanExpression()

    //Specify we want to only return groups whose name contains our search
    scanExpression.filterExpression = "contains(LowercaseName, :LowercaseGroupName)"

    //Create a scan expression to state what attributes we want to return
    scanExpression.projectionExpression = "GroupID, GroupName, LowercaseName"

    //Define our variable in the filter expression as our lowercased user input
    scanExpression.expressionAttributeValues = [":LowercaseGroupName" : lowercaseGroupName]

    dynamoDBObjectMapper.scan(GroupTableRow.self, expression: scanExpression).continueWithBlock({ (task:AWSTask!) -> AnyObject! in

        if task.result != nil {
            let paginatedOutput = task.result as! AWSDynamoDBPaginatedOutput

            //use the results
            for item in paginatedOutput.items as! [GroupTableRow] {

            }

            if ((task.error) != nil) {
                print("Error: \(task.error)")
            }
            self.queryInProgress = false
            return nil

        }

        self.queryInProgress = false
        return nil
        })
}

Answer 2

projectionExpression应该是单个逗号分隔的字符串[“ GroupID”，“ GroupName”] =>“ GroupID，GroupName”

filterExpression也是一个字符串，其文档是http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/QueryAndScan.html#FilteringResults 。 在您的情况下，我认为表达式应为“ begins_with（groupName，BeginningCharacters）”，但您可能需要对此进行试验。