Getting partial struct

[mrodriguezio]

I'm trying to get this work, but getting unexpected results.

package main

import (
    "encoding/json"
    "fmt"
    "log"

    dynamicstruct "github.com/Ompluscator/dynamic-struct"
)

func main() {
    i := dynamicstruct.NewStruct().
        AddField("AlarmName", "", `json:"alarm_name"`).
        AddField("ComparisonOperator", "", `json:"comparison_operator"`).
        AddField("EvaluationPeriods", 0, `json:"evaluation_periods"`).
        AddField("Threshold", 0, `json:"threshold"`).

        // Optional
        AddField("MetricName", "", `json:"metric_name,omitempty"`).
        AddField("Namespace", "", `json:"namespace,omitempty"`).
        AddField("Period", 0, `json:"period,omitempty"`).
        AddField("Statistic", "", `json:"statistic,omitempty"`).
        AddField("TreatMissingData", "", `json:"treat_missing_data,omitempty"`).
        Build().New()

    data := []byte(`
{
    "ActionsEnabled": null,
    "AlarmActions": ["${aws_autoscaling_policy.jenkins_cluster_scale_up_policy.id}"],
    "AlarmDescription":"CPU utilization above 5% during the last minute",
    "AlarmName":"JenkinsClusterScaleUpAlarm",
    "ComparisonOperator":"GreaterThanOrEqualToThreshold",
    "DatapointsToAlarm":null,
    "Dimensions":[{"Name":"ClusterName","Value":"jenkins-cluster"}],
    "EvaluateLowSampleCountPercentile":null,
    "EvaluationPeriods":1,
    "ExtendedStatistic":null,
    "InsufficientDataActions":null,
    "MetricName":"CPUUtilization",
    "Metrics":null,
    "Namespace":"AWS/ECS",
    "OKActions":null,
    "Period":null,
    "Statistic":"Average",
    "Threshold":20,
    "TreatMissingData":"notBreaching",
    "Unit":null
}
    `)
    var err error
    err = json.Unmarshal(data, &i)
    if err != nil {
        log.Fatal(err)
    }

    data, err = json.Marshal(i)
    if err != nil {
        log.Fatal(err)
    }

    fmt.Println(string(data))
}

Actual output:

{"namespace":"AWS/ECS","statistic":"Average","alarm_name":"","comparison_operator":"","evaluation_periods":0,"threshold":20}

Expected output:

{"namespace":"AWS/ECS","statistic":"Average","alarm_name":"JenkinsClusterScaleUpAlarm","comparison_operator":"GreaterThanOrEqualToThreshold","evaluation_periods": 1,"threshold":20,"treat_missing_data":"notBreaching","metric_name":"CPUUtilization"}

Any advice is highly appreciated :sweat_smile:!

[Ompluscator]

As you defined your json tags:

AddField("MetricName", "", `json:"metric_name,omitempty"`).
AddField("TreatMissingData", "", `json:"treat_missing_data,omitempty"`).

This means initial json input should contains keys "metric_name,omitempty" and "treat_missing_data,omitempty" and not:

"MetricName":"CPUUtilization",
"TreatMissingData":"notBreaching",

So, json package use tags for both read and write.