DynamoDB Query / Scan with combined conditions

Question

I am facing an architectural problem (or a logical one) and am not sure what is the best way to proceed. I am building a small matchmaking app to study AWS Mobile HUB and it's services and get a bit more familiar with it.

My DynamoDB contains a table with rendezvous that have the following 2 properties (and some more which are not important here):

Rendezvous

- from_gender
- interest_gender

Additionally i am using cognito and have my users have a property

User

- usr_gender
- interest_gender

Basically I want to match up users that are interested in from_gender and possess a gender of interest_gender

At the moment, User can be either male or female whereas interest_gender can be either male , female or all .

This means for a user we have 6 combinations:

Gender -> Interested In

Male -> female 
Female -> female 
Female -> all 
Male -> all 
Male -> male
Female -> male

Now it gets tricky:

Example

User A ( female interested in all ) is looking for rendezvous

She should be shown the 4 following rendezvous types:

from_gender, interest_gender

male, female
male, all
female, female
female, all

which means that I would need a query of sorts (dummycode):

rendevous.query("

(from_gender = male) AND ((interest_gender = female") OR (interest_gender = all)) OR

(from_gender = female) AND ((interest_gender = female") OR (interest_gender = all))

")

However

I have not yet found a way to get this working with query or scan as they do not support OR queries on the same property.

I tried to create indeces (using codes for the different combinations, but i did not find a way to create a queryable index for all cases), note that additionally to this interest I would also need to sort by age (between min / max) and location min/max lat, min/max long.

Are there any smart approaches using DynamoDB or am I forced to go the SQL route?

This is the query expression I have tried latest: Works

func generateScanExpression() -> AWSDynamoDBScanExpression {
    let usr = UserHelper.sharedHelper.currentUser
    let expression = AWSDynamoDBScanExpression()
    expression.limit = 10

    // Male interested in male
    if (usr?.interest == Constants.CONST_MALE) && (usr?.gender == Constants.CONST_MALE){
        expression.filterExpression = "details_fromGender = :fg AND ( match_interest = :ig OR match_interest = :all )"
        expression.expressionAttributeValues = [":fg": Constants.CONST_MALE, ":ig": Constants.CONST_MALE, ":all": Constants.CONST_ALL ]
    }
    // Female interested in female
    if (usr?.interest == Constants.CONST_FEMALE) && (usr?.gender == Constants.CONST_FEMALE){
        expression.filterExpression = "details_fromGender = :fg AND ( match_interest = :ig OR match_interest = :all )"
        expression.expressionAttributeValues = [":fg": Constants.CONST_FEMALE, ":ig": Constants.CONST_FEMALE, ":all": Constants.CONST_ALL ]
    }
    // Male interested in female
    if (usr?.interest == Constants.CONST_FEMALE) && (usr?.gender == Constants.CONST_MALE){
        expression.filterExpression = "details_fromGender = :fg AND ( match_interest = :ig OR match_interest = :all )"
        expression.expressionAttributeValues = [":fg": Constants.CONST_FEMALE, ":ig": Constants.CONST_MALE, ":all": Constants.CONST_ALL ]
    }
    // Female interested in male
    if (usr?.interest == Constants.CONST_MALE) && (usr?.gender == Constants.CONST_FEMALE){
        expression.filterExpression = "details_fromGender = :fg AND ( match_interest = :ig OR match_interest = :all )"
        expression.expressionAttributeValues = [":fg": Constants.CONST_MALE, ":ig": Constants.CONST_FEMALE, ":all": Constants.CONST_ALL ]
    }
    // Male interested in all
    if (usr?.interest == Constants.CONST_ALL) && (usr?.gender == Constants.CONST_MALE){
        expression.filterExpression = "( details_fromGender = :fg OR details_fromGender = :all ) AND match_interest = :ig"
        expression.expressionAttributeValues = [":fg": Constants.CONST_MALE, ":ig": Constants.CONST_MALE ]
    }
    // Female interested in all
    if (usr?.interest == Constants.CONST_ALL) && (usr?.gender == Constants.CONST_FEMALE){
        expression.filterExpression = "( details_fromGender = :fg OR details_fromGender = :all ) AND match_interest = :ig"
        expression.expressionAttributeValues = [":fg": Constants.CONST_MALE, ":ig": Constants.CONST_FEMALE ]
    }

    return expression
}

Answer 1

Are you absolutely certain you can't use an OR condition on a DynamoDB scan operation? I'm not saying you're wrong, I just seem to remember doing it. Here is some code I would try if I were working on the same issue:

func findMatches(interestGender: String, userGender: String, allCondition: String, map: AWSDynamoDBObjectMapper) -> AWSTask<AnyObject> {
    let scan = AWSDynamoDBScanExpression()
    scan.filterExpression = "interest_gender = :interestGender AND usr_gender = :userGender OR interest_gender = :allCondition"
    scan.expressionAttributeValues = [":interest_gender":interestGender, ":usr_gender":userGender, ":interest_gender":allCondition]

    return map.scan(Rendezvous.self, expression: scan).continue({ (task: AWSTask) -> AnyObject? in
        if (task.error != nil) {
            print(task.error)
        }

        if (task.exception != nil){
            print(task.exception)
        }

        if let result = task.result {
            return result.items as! [Rendezvous] as AnyObject?
        }

        return nil
    })

}

This would assume that you have your Rendezvous object that conforms to AWSDynamoDBObjectModel and AWSDynamoDBModeling . Try it, and let me know if it works for you, otherwise I'll delete my answer. I don't have the ability to test it for you until much later tonight, so I'm not 100% certain it will work.