Magento REST API

Not really easy to extend the REST API,  here some info which might help.

Mapping REST Operation to Magento:

// Map HTTP methods to classic CRUD verbs
$operationByMethod = array(
    'GET'    => Mage_Api2_Model_Resource::OPERATION_RETRIEVE,
    'POST'   => Mage_Api2_Model_Resource::OPERATION_CREATE,
    'PUT'    => Mage_Api2_Model_Resource::OPERATION_UPDATE,
    'DELETE' => Mage_Api2_Model_Resource::OPERATION_DELETE
);

Important insight to understand how magento decides where to route your request,
see core/Mage/Api2/Model/Mage_Api2_Model_Resource. If you don't get the desired results, check if ACTION_TYPE_xxx is set right, i.e only ACTION_TYPE_COLLECTION is allowed for OPERATION_CREATE.

public function dispatch()
{
    switch ($this->getActionType() . $this->getOperation()) {
        /* Create */
        case self::ACTION_TYPE_ENTITY . self::OPERATION_CREATE:
            // Creation of objects is possible only when working with collection
            $this->_critical(self::RESOURCE_METHOD_NOT_IMPLEMENTED);
            break;
        case self::ACTION_TYPE_COLLECTION . self::OPERATION_CREATE:
            // If no of the methods(multi or single) is implemented, request body is not checked
            if (!$this->_checkMethodExist('_create') && !$this->_checkMethodExist('_multiCreate')) {
                $this->_critical(self::RESOURCE_METHOD_NOT_IMPLEMENTED);
            }
            // If one of the methods(multi or single) is implemented, request body must not be empty
            $requestData = $this->getRequest()->getBodyParams();
            if (empty($requestData)) {
                $this->_critical(self::RESOURCE_REQUEST_DATA_INVALID);
            }
            // The create action has the dynamic type which depends on data in the request body
            if ($this->getRequest()->isAssocArrayInRequestBody()) {
                $this->_errorIfMethodNotExist('_create');
                $filteredData = $this->getFilter()->in($requestData);
                if (empty($filteredData)) {
                    $this->_critical(self::RESOURCE_REQUEST_DATA_INVALID);
                }
                $newItemLocation = $this->_create($filteredData);
                $this->getResponse()->setHeader('Location', $newItemLocation);
            } else {
                $this->_errorIfMethodNotExist('_multiCreate');
                $filteredData = $this->getFilter()->collectionIn($requestData);
                $this->_multiCreate($filteredData);
                $this->_render($this->getResponse()->getMessages());
                $this->getResponse()->setHttpResponseCode(Mage_Api2_Model_Server::HTTP_MULTI_STATUS);
            }
            break;
        /* Retrieve */
        case self::ACTION_TYPE_ENTITY . self::OPERATION_RETRIEVE:
            $this->_errorIfMethodNotExist('_retrieve');
            $retrievedData = $this->_retrieve();
            $filteredData  = $this->getFilter()->out($retrievedData);
            $this->_render($filteredData);
            break;
        case self::ACTION_TYPE_COLLECTION . self::OPERATION_RETRIEVE:
            $this->_errorIfMethodNotExist('_retrieveCollection');
            $retrievedData = $this->_retrieveCollection();
            $filteredData  = $this->getFilter()->collectionOut($retrievedData);
            $this->_render($filteredData);
            break;
        /* Update */
        case self::ACTION_TYPE_ENTITY . self::OPERATION_UPDATE:
            $this->_errorIfMethodNotExist('_update');
            $requestData = $this->getRequest()->getBodyParams();
            if (empty($requestData)) {
                $this->_critical(self::RESOURCE_REQUEST_DATA_INVALID);
            }
            $filteredData = $this->getFilter()->in($requestData);
            if (empty($filteredData)) {
                $this->_critical(self::RESOURCE_REQUEST_DATA_INVALID);
            }
            $this->_update($filteredData);
            break;
        case self::ACTION_TYPE_COLLECTION . self::OPERATION_UPDATE:
            $this->_errorIfMethodNotExist('_multiUpdate');
            $requestData = $this->getRequest()->getBodyParams();
            if (empty($requestData)) {
                $this->_critical(self::RESOURCE_REQUEST_DATA_INVALID);
            }
            $filteredData = $this->getFilter()->collectionIn($requestData);
            $this->_multiUpdate($filteredData);
            $this->_render($this->getResponse()->getMessages());
            $this->getResponse()->setHttpResponseCode(Mage_Api2_Model_Server::HTTP_MULTI_STATUS);
            break;
        /* Delete */
        case self::ACTION_TYPE_ENTITY . self::OPERATION_DELETE:
            $this->_errorIfMethodNotExist('_delete');
            $this->_delete();
            break;
        case self::ACTION_TYPE_COLLECTION . self::OPERATION_DELETE:
            $this->_errorIfMethodNotExist('_multiDelete');
            $requestData = $this->getRequest()->getBodyParams();
            if (empty($requestData)) {
                $this->_critical(self::RESOURCE_REQUEST_DATA_INVALID);
            }
            $this->_multiDelete($requestData);
            $this->getResponse()->setHttpResponseCode(Mage_Api2_Model_Server::HTTP_MULTI_STATUS);
            break;
        default:
            $this->_critical(self::RESOURCE_METHOD_NOT_IMPLEMENTED);
            break;
    }
}