record_keeping_attachment: add fields on attachment to keep connection

Author: Miracle Ayomir

record_keeping_attachment/models/attachment.py

@@ -4,7 +4,46 @@
 
 class Attachment(models.Model):
     _name = 'ir.attachment'
-    _inherit = ['ir.attachment', 'mail.thread', 'rk.document.mixin']
+    _inherit = ['ir.attachment', 'mail.thread']  # 'rk.document.mixin'
+
+    # Add document_id directly without the mixin
+    document_id = fields.Many2one(
+        'rk.document',
+        string='Document',
+        help='The record-keeping document id',
+        ondelete='restrict',
+        auto_join=True,
+        copy=False,
+        index=True
+    )
+    
+    # Related fields from document
+    classification_id = fields.Many2one(
+        'rk.classification',
+        related='document_id.classification_id',
+        store=True,
+        readonly=False
+    )
+    
+    document_type_id = fields.Many2one(
+        'rk.document.type',
+        related='document_id.document_type_id',
+        store=True,
+        readonly=False
+    )
+    
+    matter_id = fields.Many2one(
+        'rk.matter',
+        related='document_id.matter_id',
+        store=True,
+        readonly=False
+    )
+    
+    is_official = fields.Boolean(
+        related='document_id.is_official',
+        store=True,
+        readonly=False
+    )
 
     rk_file_name = fields.Char(string="Original file Name", readonly=True)