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 😅!

[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.