attachment: Add migration to fix duplicate attachment objects.

The comment in the migration explains this change in detail.

zerver/migrations/0074_fix_duplicate_attachments.py

@@ -0,0 +1,48 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.5 on 2017-04-13 22:12
+from __future__ import unicode_literals
+
+from django.db import migrations
+from django.db.backends.postgresql_psycopg2.schema import DatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+from django.db.models import Count
+
+def fix_duplicate_attachments(apps, schema_editor):
+    # type: (StateApps, DatabaseSchemaEditor) -> None
+    """Migration 0041 had a bug, where if multiple messages referenced the
+    same attachment, rather than creating a single attachment object
+    for all of them, we would incorrectly create one for each message.
+    This results in exceptions looking up the Attachment object
+    corresponding to a file that was used in multiple messages that
+    predate migration 0041.
+
+    This migration fixes this by removing the duplicates, moving their
+    messages onto a single canonical Attachment object (per path_id).
+    """
+    Attachment = apps.get_model('zerver', 'Attachment')
+    # Loop through all groups of Attachment objects with the same `path_id`
+    for group in Attachment.objects.values('path_id').annotate(Count('id')).order_by().filter(id__count__gt=1):
+        # Sort by the minimum message ID, to find the first attachment
+        attachments = sorted(list(Attachment.objects.filter(path_id=group['path_id']).order_by("id")),
+                             key = lambda x: min(x.messages.all().values_list('id')[0]))
+        surviving = attachments[0]
+        to_cleanup = attachments[1:]
+        for a in to_cleanup:
+            # For each duplicate attachment, we transfer its messages
+            # to the canonical attachment object for that path, and
+            # then delete the original attachment.
+            for msg in a.messages.all():
+                surviving.messages.add(msg)
+            surviving.is_realm_public = surviving.is_realm_public or a.is_realm_public
+            surviving.save()
+            a.delete()
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0073_custom_profile_fields'),
+    ]
+
+    operations = [
+        migrations.RunPython(fix_duplicate_attachments)
+    ]