[5.5] Use table aliases when calling self-referencing HasManyThrough relation (laravel#21883)

* Use table aliases when calling self-referencing HasManyThrough relation

*       alias the sub query table

*    fix style

Author: JayBizzle

src/Illuminate/Database/Eloquent/Relations/HasManyThrough.php

@@ -52,6 +52,13 @@ class HasManyThrough extends Relation
      */
     protected $secondLocalKey;
 
+    /**
+     * The count of self joins.
+     *
+     * @var int
+     */
+    protected static $selfJoinCount = 0;
+
     /**
      * Create a new has many through relationship instance.
      *
@@ -425,13 +432,52 @@ protected function shouldSelect(array $columns = ['*'])
      */
     public function getRelationExistenceQuery(Builder $query, Builder $parentQuery, $columns = ['*'])
     {
+        if ($parentQuery->getQuery()->from == $query->getQuery()->from) {
+            return $this->getRelationExistenceQueryForSelfRelation($query, $parentQuery, $columns);
+        }
+
         $this->performJoin($query);
 
         return $query->select($columns)->whereColumn(
             $this->getExistenceCompareKey(), '=', $this->getQualifiedFirstKeyName()
         );
     }
 
+    /**
+     * Add the constraints for a relationship query on the same table.
+     *
+     * @param  \Illuminate\Database\Eloquent\Builder  $query
+     * @param  \Illuminate\Database\Eloquent\Builder  $parentQuery
+     * @param  array|mixed  $columns
+     * @return \Illuminate\Database\Eloquent\Builder
+     */
+    public function getRelationExistenceQueryForSelfRelation(Builder $query, Builder $parentQuery, $columns = ['*'])
+    {
+        $query->from($query->getModel()->getTable().' as '.$hash = $this->getRelationCountHash());
+
+        $query->join($this->throughParent->getTable(), $this->getQualifiedParentKeyName(), '=', $hash.'.'.$this->secondLocalKey);
+
+        if ($this->throughParentSoftDeletes()) {
+            $query->whereNull($this->throughParent->getQualifiedDeletedAtColumn());
+        }
+
+        $query->getModel()->setTable($hash);
+
+        return $query->select($columns)->whereColumn(
+            $parentQuery->getQuery()->from.'.'.$query->getModel()->getKeyName(), '=', $this->getQualifiedFirstKeyName()
+        );
+    }
+
+    /**
+     * Get a relationship join table hash.
+     *
+     * @return string
+     */
+    public function getRelationCountHash()
+    {
+        return 'laravel_reserved_'.static::$selfJoinCount++;
+    }
+
     /**
      * Get the key for comparing against the parent key in "has" query.
      *

tests/Integration/Database/EloquentHasManyThroughTest.php

@@ -56,6 +56,7 @@ public function basic_create_and_retrieve()
         $notMember = User::create(['name' => str_random()]);
 
         $this->assertEquals([$mate1->id, $mate2->id], $user->teamMates->pluck('id')->toArray());
+        $this->assertEquals([$user->id], User::has('teamMates')->pluck('id')->toArray());
     }
 }