Fix HasManyThrough existence queries with same parent and through par…

…ent table (#26676)

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

@@ -480,10 +480,14 @@ protected function prepareQueryBuilder($columns = ['*'])
      */
     public function getRelationExistenceQuery(Builder $query, Builder $parentQuery, $columns = ['*'])
     {
-        if ($parentQuery->getQuery()->from == $query->getQuery()->from) {
+        if ($parentQuery->getQuery()->from === $query->getQuery()->from) {
             return $this->getRelationExistenceQueryForSelfRelation($query, $parentQuery, $columns);
         }
 
+        if ($parentQuery->getQuery()->from === $this->throughParent->getTable()) {
+            return $this->getRelationExistenceQueryForThroughSelfRelation($query, $parentQuery, $columns);
+        }
+
         $this->performJoin($query);
 
         return $query->select($columns)->whereColumn(
@@ -516,6 +520,29 @@ public function getRelationExistenceQueryForSelfRelation(Builder $query, Builder
         );
     }
 
+    /**
+     * Add the constraints for a relationship query on the same table as the through parent.
+     *
+     * @param  \Illuminate\Database\Eloquent\Builder  $query
+     * @param  \Illuminate\Database\Eloquent\Builder  $parentQuery
+     * @param  array|mixed  $columns
+     * @return \Illuminate\Database\Eloquent\Builder
+     */
+    public function getRelationExistenceQueryForThroughSelfRelation(Builder $query, Builder $parentQuery, $columns = ['*'])
+    {
+        $table = $this->throughParent->getTable().' as '.$hash = $this->getRelationCountHash();
+
+        $query->join($table, $hash.'.'.$this->secondLocalKey, '=', $this->getQualifiedFarKeyName());
+
+        if ($this->throughParentSoftDeletes()) {
+            $query->whereNull($hash.'.'.$this->throughParent->getDeletedAtColumn());
+        }
+
+        return $query->select($columns)->whereColumn(
+            $parentQuery->getQuery()->from.'.'.$this->localKey, '=', $hash.'.'.$this->firstKey
+        );
+    }
+
     /**
      * Get a relationship join table hash.
      *

tests/Integration/Database/EloquentHasManyThroughTest.php

@@ -4,6 +4,7 @@
 
 use Illuminate\Support\Facades\Schema;
 use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\SoftDeletes;
 use Illuminate\Tests\Integration\Database\DatabaseTestCase;
 
 /**
@@ -27,6 +28,17 @@ public function setUp()
             $table->integer('owner_id')->nullable();
             $table->string('owner_slug')->nullable();
         });
+
+        Schema::create('categories', function ($table) {
+            $table->increments('id');
+            $table->integer('parent_id')->nullable();
+            $table->softDeletes();
+        });
+
+        Schema::create('products', function ($table) {
+            $table->increments('id');
+            $table->integer('category_id');
+        });
     }
 
     public function test_basic_create_and_retrieve()
@@ -83,6 +95,21 @@ public function test_has_self_custom_owner_key()
 
         $this->assertEquals(1, $users->count());
     }
+
+    public function test_has_same_parent_and_through_parent_table()
+    {
+        Category::create();
+        Category::create();
+        Category::create(['parent_id' => 1]);
+        Category::create(['parent_id' => 2])->delete();
+
+        Product::create(['category_id' => 3]);
+        Product::create(['category_id' => 4]);
+
+        $categories = Category::has('subProducts')->get();
+
+        $this->assertEquals([1], $categories->pluck('id')->all());
+    }
 }
 
 class User extends Model
@@ -129,3 +156,22 @@ class Team extends Model
     public $timestamps = false;
     protected $guarded = [];
 }
+
+class Category extends Model
+{
+    use SoftDeletes;
+
+    public $timestamps = false;
+    protected $guarded = [];
+
+    public function subProducts()
+    {
+        return $this->hasManyThrough(Product::class, self::class, 'parent_id');
+    }
+}
+
+class Product extends Model
+{
+    public $timestamps = false;
+    protected $guarded = [];
+}