I am developing an Asp.Net Core MVC web application.

One of the Razor views has three drop-down lists.

* The first drop-down list "Study DDL" is populated by the ViewModel data which is passed to the view. * The secondary drop-down list "Scenario DDL" is populated using a JQuery and Ajax call based on the selected value from the first drop-down list (cascading). * The third drop-down list "Project DDL" is a multi-select DDL.

View Code: -------------

<select class="form-control col-md-8" onchange="GetScenario(this.value);"
		asp-for="StudyId"
		asp-items="@(new SelectList(@Model.StudyList, "StudyId", "StudyName"))">
	<option value="">--Please Select--</option>
</select>

<select id="ddlScenario" class="form-control col-md-8" name="ddlScenario"
		asp-for="ScenarioId">
	<option value="">--Please Select--</option>
</select>


<select class="form-control col-md-8" multiple size="7"
		asp-for="ProjectId1ToMany"
		asp-items="@(new SelectList(@Model.ProjectList, "ProjectId", "ProjectName"))">
	<option value="">--Please Select--</option>
</select>

Javascript/JQuery code: ---------------------------

function GetScenario(_studyId) {
	var url = "/MappingStudy/GetScenarioByStudyId/";

	$.ajax({
		url: url,
		data: { selectedStudyId: _studyId },
		cache: false,
		type: "POST",
		success: function (data) {
			
			var markup = "<option value=''>--Please Select--</option>";
			for (var x = 0; x < data.length; x++) {
				markup += "<option value=" + data[x].value + ">" + data[x].text + "</option>";
			}

			$("#ddlScenario").html(markup);			
		},
		failure: function (response) {
			alert("fail : " + response.responseText);
		},
		error: function (response) {
			alert("error : " + response.responseText);
		}
	});
}

Repository Code: -------------------

IEnumerable<HdrFdpScenarios> GetScenarioDDL();

public IEnumerable<HdrFdpScenarios> GetScenarioDDL()
{
	return context.HdrFdpScenarios;
}

IEnumerable<HdrPearlProject> GetProjectDDL();

public IEnumerable<HdrPearlProject> GetProjectDDL()
{
	return context.HdrPearlProject;
}

ViewModel class: -----------------

namespace SLB.ETLWebApp.ViewModel
{
    public class MappingStudyViewModel
    {
        public string StudyId { get; set; }
        public List<HdrFdpStudy> StudyList { get; set; }

        public string ScenarioId { get; set; }
        public List<HdrFdpScenarios> ScenarioList { get; set; }

        public string[] ProjectId1ToMany { get; set; }
        public List<HdrPearlProject> ProjectList { get; set; }
        
        public int Id { get; set; }
        public int MappingId { get; set; }
        public IEnumerable<MappingDetails> MappingDetails { get; set; }
		
		public IEnumerable<Mapping> Mapping { get; set; }
    }
}

Controller Code: -----------------

public List<HdrFdpScenarios> BindScenarioDDL()
{
	List<HdrFdpScenarios> scenarioList = new List<HdrFdpScenarios>();
	scenarioList = _repository.GetScenarioDDL().ToList();

	return scenarioList;
}

public JsonResult GetScenarioByStudyId(string selectedStudyId)
{
	List<HdrFdpScenarios> scenarioList = new List<HdrFdpScenarios>();
	scenarioList = BindScenarioDDL().Where(m => m.StudyId == selectedStudyId).ToList();
	scenarioList.Select(x => new { ScenarioId = x.ScenarioId, ScenarioName = x.ScenarioDisplayId + " | " + x.ScenarioName });  //concatinate 2 columns data in DDL

	var objScenarioList = new SelectList(scenarioList, "ScenarioId", "ScenarioName");
	ViewBag.ScenarioList = objScenarioList;

	return new JsonResult(objScenarioList);
}

public List<HdrPearlProject> BindProjectDDL()
{
	List<HdrPearlProject> projectList = new List<HdrPearlProject>();         
	projectList = _repository.GetProjectDDL().ToList();

	return projectList;
}

public string GetStudyIdByStudyName(string studyName)
{
	var data = (from x in BindStudyDDL().ToList()
				where x.StudyName == studyName
				select x.StudyId).FirstOrDefault();

	return data;
}

[HttpGet]
public IActionResult Upsert(int id, int mappingId)
{
	MappingStudyViewModel mappingStudyViewModel = new MappingStudyViewModel();

	mappingStudyViewModel.StudyList = BindStudyDDL();
	mappingStudyViewModel.ProjectList = BindProjectDDL();

	if (id == 0)
	{
		//Create
		return View(mappingStudyViewModel);
	}
	else
	{
		//Update
		var objMD = _repository.GetAllMappingDetails().Where(m => m.Id == id).FirstOrDefault();
		var objM = _repository.GetAllMapping().Where(m => m.MappingId == mappingId).FirstOrDefault();

		if (objM.MappingType == "1To1")
		{
			mappingStudyViewModel.Id = id;
			mappingStudyViewModel.MappingId = mappingId;
			mappingStudyViewModel.StudyId = GetStudyIdByStudyName(objMD.FdpStudy);
			
			//mappingStudyViewModel.ScenarioList = BindScenarioDDL().Where(m => m.StudyId == mappingStudyViewModel.StudyId).ToList();
			//mappingStudyViewModel.ScenarioList.Select(x => x.ScenarioDisplayId + " | " + x.ScenarioName);
			//var objScenarioList = new SelectList(mappingStudyViewModel.ScenarioList, "ScenarioId", "ScenarioName");
			//ViewBag.ScenarioList = objScenarioList;                    
			//mappingStudyViewModel.ScenarioId = ViewBag.ScenarioList;

			mappingStudyViewModel.ScenarioId =  ?? // how to bind and fetch Scenario DDL value from DB, based on selecetd StudyId of Study DDL ?
		}
		else
		{		
			mappingStudyViewModel.ProjectId1ToMany = ?? // how to bind multiple value from Database in multiselect dropdown ?			
		}
		return View(mappingStudyViewModel);
	}
}

I am facing 2 issues: 1. how to fetch and bind "Scenario DDL" value from DB, based on the selected value of "Study DDL"? 2. how to fetch and bind multiple values from Database in multi-select dropdown list?