dotnet/AspNetCore.Docs · error · Error

There already is an object with the key that you specified.

Error message

There already is an object with the key that you specified.

What it means

knockout.mapping.js's `mappedCreate` checks `mappedIndexOf(value) !== -1` and throws if the item's key already exists in the mapped observable array. Each mapped array is keyed by a callback (often an id property), and uniqueness is enforced on create.

Source

Thrown at aspnetcore/mvc/controllers/testing/samples/3.x/TestingControllersSample/src/TestingControllersSample/wwwroot/js/knockout.mapping.js:542

					var arrayOfKeys = filterArrayByKey(arrayOfValues, keyCallback);
					return mappedRootObject.destroy(function (item) {
						return ko.utils.arrayIndexOf(arrayOfKeys, keyCallback(item)) != -1;
					});
				}

				mappedRootObject.mappedIndexOf = function (item) {
					var keys = filterArrayByKey(mappedRootObject(), keyCallback);
					var key = keyCallback(item);
					return ko.utils.arrayIndexOf(keys, key);
				}

				mappedRootObject.mappedGet = function (item) {
					return mappedRootObject()[mappedRootObject.mappedIndexOf(item)];
				}

				mappedRootObject.mappedCreate = function (value) {
					if (mappedRootObject.mappedIndexOf(value) !== -1) {
						throw new Error("There already is an object with the key that you specified.");
					}

					var item = hasCreateCallback() ? createCallback(value) : value;
					if (hasUpdateCallback()) {
						var newValue = updateCallback(item, value);
						if (ko.isWriteableObservable(item)) {
							item(newValue);
						} else {
							item = newValue;
						}
					}
					mappedRootObject.push(item);
					return item;
				}
			}

			var currentArrayKeys = filterArrayByKey(ko.utils.unwrapObservable(mappedRootObject), keyCallback).sort();
			var newArrayKeys = filterArrayByKey(rootObject, keyCallback);

View on GitHub (pinned to c67a80103a)

Solutions

1. Before creating, check `if (myMappedArray.mappedIndexOf(newItem) === -1) myMappedArray.mappedCreate(newItem);`.
2. Use `ko.mapping.fromJS(newData, { key: x => x.id }, myMappedArray)` which updates existing keys in place rather than creating duplicates.
3. Dedupe the incoming dataset by the key property before mapping.
4. Verify the key callback returns a genuinely unique value per item.

Example fix

// before
myMappedArray.mappedCreate(newItem);

// after
if (myMappedArray.mappedIndexOf(newItem) === -1) {
  myMappedArray.mappedCreate(newItem);
}

function safeCreate(mappedArray, value) {
  if (mappedArray.mappedIndexOf(value) !== -1) {
    throw new Error('Duplicate key — use fromJS with a key to update instead');
  }
  mappedArray.mappedCreate(value);
}

function isUniqueKey(mappedArray, value) {
  return mappedArray.mappedIndexOf(value) === -1;
}

try {
  myArray.mappedCreate(item);
} catch (e) {
  if (/already is an object/.test(e.message)) {
    ko.mapping.fromJS([item], { key: x => x.id }, myArray);
  } else throw e;
}

When it happens

Trigger: Calling `myMappedArray.mappedCreate(newItem)` where `mapKey(newItem)` matches an existing element's key. Also triggered by importing data with duplicate ids into a keyed mapping.

Common situations: Server data containing duplicate primary keys; re-running fromJS over an array that already has the items without a key update path; wrong key callback that collapses distinct items to the same key.

Related errors

• When calling ko.mapping.toJS, pass the object you want to co
• When calling ko.update*, the key '${key}' was not found!
• When calling ko.fromJS, pass the object you want to convert.
• When calling ko.mapping.toJS, pass the object you want to co
• ko.mapping.defaultOptions().ignore should be an array.

AI-assisted analysis of dotnet/AspNetCore.Docs@c67a80103a (2026-08-13). Data as JSON: /api/errors/0fd350e0595ea6ed. Report an issue: GitHub.