Summing values of an object's map gives me errors

问题

A SubjectTeacherPeriod has a num_attribute_map, which is a map that maps certain attributes (such as "boringness") with their respective scores. I use the following code to sum attributes (such as "boringness") over each day of the week.

But a certain line causes an error.

rule "insertAttributeDayTotal"
        //salience 1 // Do these rules first (optional, for performance)
    when
    $sum_regression_constraint : SumRegressionConstraint(
                    $class : class_,
                    $attribute : attribute//,
                    //$weight : weight;
                   )
    $day_of_week : DayOfWeek()
    $attribute_day_total : Number() from accumulate(
        SubjectTeacherPeriod(
            //period != null,
                period.class_ == $class,
            period.dayOfWeek == $day_of_week,
                $total : num_attribute_map[$attribute] //PROBLEM LINE
            ),
            sum($total)
        )

    then
    //System.out.println("BUCKET TOTAL "+$id+" "+$bucket_total.intValue());
        insertLogical(new AttributeDaySum($class, $attribute, $day_of_week, $attribute_day_total.intValue()));
end

The error is:

jesvin@Jesvin-Technovia:~/dev/drools/timetabler$ java -server in.co.technovia.timetabler.TimeTableApp
Exception in thread "main" java.lang.IllegalStateException: There are errors in the scoreDrl's:
Variables can not be used inside bindings. Variable [$attribute] is being used in binding 'num_attribute_map[$attribute]' : [Rule name='insertAttributeDayTotal']

Rule Compilation error : [Rule name='insertAttributeDayTotal']
    in/co/technovia/timetabler/domain/Rule_insertAttributeDayTotal_bb39fd28b3c8457cb8d86fc15b34a0e7.java (7:905) : Syntax error on token "null", invalid Type
    in/co/technovia/timetabler/domain/Rule_insertAttributeDayTotal_bb39fd28b3c8457cb8d86fc15b34a0e7.java (9:1050) : $total cannot be resolved

SubjectTeacherPeriod has the curious num_attribute_map so that I can define attributes at runtime. If I wanted a boringness (int) attribute for SubjectTeacherPeriod, I can do num_attribute_map.put("boringness",1) instead of adding a new attribute to SubjectTeacherPeriod.

A SumRegressionConstraint cares about a particular $attribute. That attribute's value is stored in num_attribute_map of SubjectTeacherPeriod. I want to access num_attribute_map[$attribute] but this problem shows up.

What am I doing wrong?

Is there any other way to get dynamic attributes to work?

回答1:

At the moment, you can't bind a variable to expressions, only to field names. So instead of binding it to:

$total : num_attribute_map[$attribute]

Bind it to:

$total : num_attribute_map

Then, you can use the expression on the function. If you are using the MVEL dialect:

sum( $total[$attribute] )

Or if you are using the java dialect:

sum( $total.get($attribute) )

来源：https://stackoverflow.com/questions/9444470/summing-values-of-an-objects-map-gives-me-errors

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