MAGETWO-50123: Login failed after new custom attribute was added

- removing SelectBackend class and fixing the problem from the javascript side by ensuring that select is posted even on empty value

Author: cpartica

app/code/Magento/Catalog/Ui/DataProvider/Product/Form/Modifier/Eav.php

@@ -523,11 +523,11 @@ private function getPreviousSetAttributes()
     }
 
     /**
-     * Check is product already exists or we trying to create one
+     * Check is product already new or we trying to create one
      *
      * @return bool
      */
-    private function isProductExists()
+    private function isProductNew()
     {
         return (bool) $this->locator->getProduct()->getId();
     }
@@ -552,7 +552,7 @@ private function isProductHasValueForAttribute(ProductAttributeInterface $attrib
      */
     private function isShowDefaultValue(ProductAttributeInterface $attribute)
     {
-        if (!$this->isProductExists()) {
+        if (!$this->isProductNew()) {
             return true;
         } elseif ($attribute->getIsRequired() && !$this->isProductHasValueForAttribute($attribute)) {
             return true;

app/code/Magento/Eav/Model/Entity/Attribute.php

@@ -167,15 +167,6 @@ protected function _getDefaultBackendModel()
                 break;
         }
 
-        // we should override default behaviour of Magento\Framework\Exception\LocalizedException\AbstractBackend
-        // but can't do this in abstract model and can't update DB records because of backward compatibility
-        if ($this->getFrontendInput() == 'select'
-            && !$this->getIsRequired()
-            && !$this->getBackendModel()
-        ) {
-            return \Magento\Eav\Model\Entity\Attribute\Backend\SelectBackend::class;
-        }
-
         return parent::_getDefaultBackendModel();
     }
 

app/code/Magento/Eav/Model/Entity/Attribute/Backend/SelectBackend.php

@@ -163,6 +163,10 @@ define([
             this.observe('options')
                 .setOptions(this.options());
 
+            if (_.isUndefined(this.value()) && !this.default) {
+                this.clear();
+            }
+
             return this;
         },
 
@@ -194,7 +198,7 @@ define([
         },
 
         /**
-         * Matches specfied value with existing options
+         * Matches specified value with existing options
          * or, if value is not specified, returns value of the first option.
          *
          * @returns {*}