[11.x] Added for method to Eloquent models

src/Illuminate/Database/Eloquent/Model.php

@@ -577,6 +577,24 @@ public function forceFill(array $attributes)
         return static::unguarded(fn () => $this->fill($attributes));
     }
 
+    /**
+     * Set the foreign key for a relationship to another model.
+     *
+     * @param  \Illuminate\Database\Eloquent\Model  $model
+     * @param  string|null  $relationship
+     * @return $this
+     */
+    public function for($model, $relationship = null)
+    {
+        $relationship ??= Str::camel(class_basename($model));
+
+        $foreignKey = $this->{$relationship}()->getForeignKeyName();
+
+        $this->forceFill([$foreignKey => $model->getKey()]);
+
+        return $this;
+    }
+
     /**
      * Qualify the given column name by the model's table.
      *

tests/Database/DatabaseEloquentModelTest.php

@@ -3178,6 +3178,29 @@ public function testCollectedByAttribute()
 
         $this->assertInstanceOf(CustomEloquentCollection::class, $collection);
     }
+
+    public function testForCanSetForeignKeysForRelationships()
+    {
+        $post = new Post();
+        $post->id = 123;
+
+        $user = new DatabaseEloquentModelTestUser();
+        $user->id = 456;
+
+        $comment = (new Comment());
+        $this->addMockConnection($comment);
+
+        // Set some data so we can make sure "for" isn't clearing it.
+        $comment->content = 'Hello';
+
+        $comment
+            ->for($post)
+            ->for($user, 'user');
+
+        $this->assertSame($comment->content, 'Hello');
+        $this->assertSame($comment->post_id, 123);
+        $this->assertSame($comment->the_user_id, 456);
+    }
 }
 
 class EloquentTestObserverStub
@@ -3946,3 +3969,28 @@ class EloquentModelWithCollectedByAttribute extends Model
 class CustomEloquentCollection extends Collection
 {
 }
+
+class DatabaseEloquentModelTestUser extends Model
+{
+    protected $guarded = [];
+}
+
+class Post extends Model
+{
+    protected $guarded = [];
+}
+
+class Comment extends Model
+{
+    protected $guarded = [];
+
+    public function post(): BelongsTo
+    {
+        return $this->belongsTo(Post::class);
+    }
+
+    public function user(): BelongsTo
+    {
+        return $this->belongsTo(DatabaseEloquentModelTestUser::class, 'the_user_id');
+    }
+}