angular ui grid grouping display first element in the group row

Question

I need to display the first value in a group in the group row so first i tried to display a constant using aggregation, I have this column def

     name: 'Stack',
    displayName: 'Stack',
    grouping: { groupPriority: 1 },
    treeAggregation: { type:this.uiGridGroupingConstants.aggregation.CUSTOM },
    customTreeAggregationFn: this.aggregateService.accumulateNumValue,
    customTreeAggregationFinalizerFn: this.aggregateService.medianFinalize,
    visible: true,
    minWidth: '70',

Grouping is done using another column which is hidden, I tried to using custom aggregationTree with finalize method doing only

        if ( angular.isUndefined(aggregation.stats.accumulator) ){
            aggregation.stats.accumulator = [];
        }

this piece of code throws an error

 TypeError: Cannot read property 'accumulator' of undefined
at GridColumn.AggregationService.medianFinalize [as customTreeAggregationFinalizerFn] (http://localhost:8080/bundle.js:135388:55)
at Object.finaliseAggregation (http://localhost:8080/bundle.js:74042:28)
at http://localhost:8080/bundle.js:74073:20
at Array.forEach (native)
at Object.finaliseAggregations (http://localhost:8080/bundle.js:74072:36)
at createNode (http://localhost:8080/bundle.js:73647:24)
at Array.forEach (native)
at Object.createTree (http://localhost:8080/bundle.js:73679:25)
at Grid.treeRows (http://localhost:8080/bundle.js:73544:39)
at startProcessor (http://localhost:8080/bundle.js:52026:34)

What I do not understand how this throws an error, I removed the 3 lines of code and the error no longer exists but i clearly need to access the tats.accumulator as this is the array where i push my data in the aggregation function.

Answer 1

I was able to solve the issue the problem was with the aggregation function not the finalizer function, in my aggregation service I had this function

 public accumulateNumValue (aggregation, fieldValue, value) {

        if (angular.isUndefined(aggregation.stats) ){
            aggregation.stats = {sum: 0};
        }
        if ( angular.isUndefined(aggregation.stats.accumulator) ){
            aggregation.stats.accumulator = [];
        }

        aggregation.stats.accumulator.push(value);

    }

I was pushing the value not fieldValue, i just started pushing the field value and the accumulate function is working perfectly now.

     public accumulateNumValue (aggregation, fieldValue, value) {

        if (angular.isUndefined(aggregation.stats) ){
            aggregation.stats = {sum: 0};
        }
        if ( angular.isUndefined(aggregation.stats.accumulator) ){
            aggregation.stats.accumulator = [];
        }

        aggregation.stats.accumulator.push(fieldValue);

    }

and to display the first element in the group all you need in your finalizer function is to assign "aggregation.rendered" to the first element of the array

     public medianFinalize(aggregation) {
        if (angular.isUndefined(aggregation.stats) ) {
            aggregation.stats={accumulator : []};
        }
        if(aggregation.stats.accumulator.length >0) {

            aggregation.rendered = aggregation.stats.accumulator[0];
        }
    }