Datastore TTL fix

Constant expression contains invalid operations

Author: iamacarpet

src/Eloquent/Model.php

@@ -68,13 +68,6 @@ abstract class Model extends BaseModel
      */
     protected $excludeFromIndexes = [];
 
-    /**
-     * A list of attributes to exclude from the default indexing strategy.
-     *
-     * @var null|CarbonInterval
-     */
-    protected $expireAfter;
-
     /**
      * The primary key for the datastore should be "id".
      *
@@ -491,8 +484,10 @@ public function refresh()
      */
     public function updateTimestamps()
     {
-        if (null !== $this->expireAfter && $this->expireAfter instanceof CarbonInterval) {
-            $time = $this->freshTimestamp()->add($this->expireAfter);
+        $expireAfter = $this->getExpireAfterInterval();
+
+        if (null !== $expireAfter && $expireAfter instanceof CarbonInterval) {
+            $time = $this->freshTimestamp()->add($expireAfter);
 
             $expireAtColumn = $this->getExpireAtColumn();
 
@@ -518,6 +513,16 @@ public function setExpireAt($value)
         return $this;
     }
 
+    /**
+     * Get the expiry interval for the record.
+     *
+     * @return null|CarbonInterval
+     */
+    public function getExpireAfterInterval()
+    {
+        return null;
+    }
+
     /**
      * Get the name of the "expire at" column.
      *