recovering-deleted-wagtail-pages/slides.md

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title: Recovering deleted Wagtail pages and Django models
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# Recovering [deleted]{style="color: #fd5765"} Wagtail pages and/or Django models

### Jake Howard{.mt-10}

<ul class="list-none! text-sm [&>li]:m-0! mt-1 uppercase">
  <li>Senior Systems Engineer @ Torchbox</li>
  <li>Core Team, Security Team & Performance Working Group @ Wagtail</li>
</ul>

<ul class="list-none! text-sm [&>li]:m-0! mt-3">
  <li><mdi-earth /> theorangeone.net</li>
  <li><mdi-twitter /> @RealOrangeOne</li>
  <li><mdi-github /> @RealOrangeOne</li>
  <li><mdi-mastodon /> @jake@theorangeone.net</li>
</ul>

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# Setting the scene

<!--
- People usually use Wagtail as a website or blog
- At Torchbox, we use it for internal documentation ("intranet")
  - Processes
  - Company information
  - Links to other places etc
- Been around for a while
  - Slowly growing
- In 2022, we restructured the content
  - Make it easier to find things
  - Remove duplication
- This didn't quite go to plan
- One afternoon, I was looking to reference a process, and couldn't find it
  - Turns out, the entire "Sysadmin" section had completely vanished
-->

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# Site history report

<!--
- First step: Understanding what happened
  - Wagtail has me covered there
  - The site history report!
- Fortunately, Wagtail showed _almost_ exactly what had happened
  - One staff member deleted the "Sysadmin" section a few days before
    - Which deleted every page under it
      - All 105 of them
-->

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<!--
- I messaged the person, to better understand what happened
  - Assuming they didn't mean to delete all that content
  - Hanlon's Razor
- They'd made a new "Sysadmin" section a while ago, before switching strategy to move pages in the existing tree
  - They then deleted the wrong one
  - Sure, Wagtail shows a confirmation when you're deleting pages, but when you're deleting a lot of pages, and expecting to delete pages, you might not read the message perfectly
- "Radical reorganisation" my boss called it
- With the content gone, I had to restore from backups.
-->

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# Restoring from backups

<!--
- Our intranet is a living document, it gets updated fairly often
- Rolling back the entire system almost 2 days would have meant potentially losing critical changes
  - Not to mention people's time they spent making the changes
- It'd be annoying, but we _could_ do it, but I'd rather another solution
-->

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# _Partially_ restoring from backups

<!--
- Ideally, what I needed was to restore only the sysadmin pages, leaving all others completely untouched.
- Using a few tricks of Django and Wagtail internals, it's absolutely possible, and I did it
  - With 0 downtime, too!
-->

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## 1.
# Spin up a database backup

<!--
- We backup our intranet nightly, so I downloaded a backup from before the incident
- Spin up the codebase locally so I can interrogate it
-->

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## 2.
# Locate the page models

<div class="pt-5 text-left">

```python
from wagtail.models import Page

sysadmin_page = Page.objects.get(id=91)

child_pages = sysadmin_page.get_descendants()
```

</div>

<!--
- Behind the scenes, Wagtail pages are a tree, implemented using `django-treebeard`.
- When a page is deleted, treebeard is the one who finds all the child pages and deletes them too
  - And then Django and the database deal with cascading the delete
-->

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## 3.
# Locate what was deleted

<div class="pt-5 text-left">

```python
from django.contrib.admin.utils import NestedObjects

collector = NestedObjects()
collector.collect(list(child_pages) + [sysadmin_page])
```

</div>

<!--
- This is where the magic happens
- Deleting a page deletes more than just a page
  - The specific model
  - Revisions
  - Related models
  - Through tables
- `get_descendants` won't get all those
  - Calling `.delete` gives you the number of objects, and it's quite a lot
- If you've ever used the Django admin, you know it's capable of finding every model instance before a delete
  - That's implemented with an undocumented but simple to use API
- Yes, that's really it. It doesn't delete the models, it just tells us what _would_ be if we triggered a delete.
-->

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## 4.
# Serialize

<div class="pt-5 text-left">

```python {all|3-5}
from django.core import serializers

class NoM2MSerializer(Serializer):
    def handle_m2m_field(self, obj, field):
        pass

def get_model_instances():
    for qs in collector.data.values():
        yield from qs

with open("deleted-models.json", "w") as f:
    NoM2MSerializer().serialize(
        get_model_instances(),
        stream=f
    )
```

</div>

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<!--
- `collector.data` now contains all the model instances which were deleted, in memory on my laptop
  - My laptop isn't what's running production
- Need to serialize the models so they can be then be loaded onto production
  - If you're thinking of fixtures, you're right
- Django's fixtures create a JSON representation of a model, so they can be saved in 1 location and loaded into another
  - Mostly useful for complex test fixtures (hence the name), but generally useful for cases like this
- [click]`NoM2MSerializer` is a bit special
  - When Django serializes a model with a m2m which doesn't use a custom table, it inlines the definition, because it's easier to work with
  - However, `NestedObjects` still finds these through tables, and tries to load them separately
  - Resulting in duplicate objects and referential integrity issues
  - Instead, we exclude them, knowing we picked them up explicitly with `NestedObjects`
-->

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## 4a.
# [De]{.italic}serialize

### `manage.py loaddata`

<!--
- We have a JSON file, the inverse is just `manage.py loaddata`
-->

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# `restore-deleted-pages.py`

```python {all}{lines:true}
from django.contrib.admin.utils import NestedObjects
from django.core import serializers

from wagtail.models import Page

class NoM2MSerializer(Serializer):
    def handle_m2m_field(self, obj, field):
        pass

sysadmin_page = Page.objects.get(id=91)

child_pages = sysadmin_page.get_descendants()

collector = NestedObjects()
collector.collect(list(child_pages) + [sysadmin_page])

def get_model_instances():
    for qs in collector.data.values():
        yield from qs

with open("deleted-models.json", "w") as f:
    NoM2MSerializer().serialize(
        get_model_instances(),
        stream=f
    )
```

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<!--
- If we combine it all together, this is the big script we end up with
-->

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### 5.
# **Test!**

<!--
- For what I hope are obvious reasons, this needed to be tested!
- I deleted the page through the wagtail admin locally, and then restored them to confirm they're all the same
- I'm glad I did, because there was an issue: Search indexes
  - The search index objects (we use postgres) were picked up by `NestedObjects`
  - They didn't like being restored
  - So I skipped them and moved on, knowing I'd just rebuild the index later.
-->

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### 6.
# Showtime!

<v-clicks>

1. Backup! ✅
2. Send `deleted-models.json` to server ✅
3. `loaddata` ✅
4. `checktree` ✅
5. `update_index` ✅
6. `rebuild_reference_index` ✅

</v-clicks>

<!--
- The tense bit
- Once I was happy, I ran the same steps on production
- Our intranet runs on Heroku, so I had to do a few dances to get the JSON file up there.
- [click]Before I began, I did a backup
  - Because I'm a good sysadmin
- [click]With the data file in place, I crossed everything[click] and ran `loaddata`
  - Pages popped up in the admin as if they never left
- [click]`checktree` worked.
- [click]`update_index` worked.
- [click] As did `rebuild_reference_index`
  - The new pages were now findable and searchable
-->

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# Conclusion

<!--
- With a few hours work, the pages were back
  - There was no downtime
  - No content freeze
  - No data loss
- Most people didn't even know there was an issue
- I've used this trick a few times in my career, for both Wagtail and plain Django sites
  - Ironically, just a few weeks after the blog post was published
  - Works identically for any Django project, so long as you know how to reconstruct the delete query.
- Hopefully this helps you out as much as it has me!
-->

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  <img src="/blog-post-qrcode.png" width="190px" />
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https://wagtail.org/blog/recovering-deleted-pages-and-models/