Use knockout to dynamically populate a dropdown menu with data retrieved from a database through an AJAX request

I am working on a functionality where I have two drop down lists. The first drop down list contains a few values, and based on the selection of a value from this list, I need to populate the second drop down list with values retrieved from a database.

To achieve this, I am using knockout.js along with an AJAX query that fetches data from the database and processes it in my JavaScript code.

When I select an item from the first drop down list, I can confirm that the correct values for the second drop down list are retrieved by checking alerts or the console log. Everything seems to be functioning properly in terms of sending the selected value to the database via AJAX and receiving the corresponding list of values.

However, I am facing an issue where the second drop down list is not getting populated with these values. It appears to be related to how I am binding the data with knockout.js, but I am unsure about the exact steps to take next.

Any assistance on this matter would be greatly appreciated. Thank you!

self.getSomeValuesFromDb = function (url, valueToSendToDb) {
        $.ajax({
            type: 'POST',
            dataType: 'json',
            url: url,
            data: {
                valueToSendToDb: valueToSendToDb
            },
            success: function (response) {
                self.valueObsArray = ko.observableArray([]);
                self.selected_value = ko.observable();
                $.each(response, function (index, theValue) {
                    self.valueObsArray.push(response[index]);
                });
                alert(self.valueObsArray());
            },
            error: function (xhr, ajaxOptions, thrownError) {
                console.log("There has been an error retrieving the values from the database.");
            }     
        });





<div class="form-group">
<label class="col-sm-3 col-md-3 control-label">DDL 2:</label>
<div class="col-sm-9 col-md-9" data-bind="with: $parent.popup.selected_valueDropDownList1">
<select class="form-control" data-bind="options: $root.getSomeValuesFromDb('/SomeValue/Test','theValue'), value: $root.valueObsArray, optionsCaption: 'Select a value'"></select></div></div>

Answer №1

To make your code work correctly, ensure that you initialize valueObsArray outside the getSomeValuesFromDb function. Inside the function, reset the valueObsArray to an empty array using the following line of code:

valueObsArray([]);

Your updated code should appear as follows:

self.valueObsArray = ko.observableArray([]);
self.getSomeValuesFromDb = function (url, valueToSendToDb) {
    $.ajax({
        type: 'POST',
        dataType: 'json',
        url: url,
        data: {
            valueToSendToDb: valueToSendToDb
        },
        success: function (response) {
            self.valueObsArray([]);
            self.selected_value = ko.observable();
            $.each(response, function (index, theValue) {
                self.valueObsArray.push(response[index]);
            });
            alert(self.valueObsArray());
        },
        error: function (xhr, ajaxOptions, thrownError) {
            console.log("There has been an error retrieving the values from the database.");
        }     
    });

Alternatively, you can also utilize a pureComputed function to perform an AJAX call and populate the second dropdown based on the selection in the first dropdown:

var simpleListModel = function() {
  var self = this;
  self.firstOptions = ko.observable(["", "Apple", "Orange", "Mango"]);
  self.selectedFirstOption = ko.observable("");
  self.secondOptions = ko.pureComputed(function() {
    if(self.selectedFirstOption() === "") return [];
    else {
      // perform AJAX call here
      var number = 0;
      return ko.utils.arrayMap(self.firstOptions(), function(item) {
        return self.selectedFirstOption() + number++;
      });
    }
  }, this);
};

ko.applyBindings(simpleListModel); 
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.2.0/knockout-min.js"></script>

<div>
  <select data-bind="options: firstOptions, value: selectedFirstOption"></select>
</div>
<div>
  <select data-bind="options: secondOptions"></select>
</div>

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