Problems with using dynamic media_folder name

[smashercosmo]

Describe the bug
This is how I define my posts collection

collections:
  - label: Posts
    label_singular: post
    name: posts
    folder: content/posts
    media_folder: '/{{media_folder}}/posts/{{filename}}'
    public_folder: '{{public_folder}}/posts/{{filename}}'
    format: frontmatter
    create: true
    identifier_field: title
    fields:
      - {label: Title, name: title, widget: string}
      - {label: Cover image, name: cover, widget: image, required: false}
      - {label: Body, name: body, widget: markdown}

as you can see media_folder name is based on the filename. But the problem is that when you create post file doesn't exist, so images in markdown is written with wrong path.

How to reproduce
Here is reproduction repository
https://github.com/smashercosmo/folder-collections-media-and-public-folder-issue

1. run npm run develop
2. open http://localhost:8000/admin/
3. press new post
4. fill the form and upload an image
5. press publish

Expected behavior
you should see correct image path in frontmatter

---
cover: /img/posts/new-post-name/image.jpg
---

Actual behavior
you will see this instead

---
cover: /img/posts/image.jpg
---

Applicable Versions:

• Netlify CMS version: 2.12.12

[jeremyzilar]

As a side note, thank you for adding this. I didn't realize that media_folder and public_folder could be added as additional params within a collection. 🙌

[erezrokah]

Hi @jeremyzilar, we've added it recently so its under beta feature https://www.netlifycms.org/docs/beta-features/#folder-collections-media-and-public-folder.
We haven't documented it, but you can also configure it per field (https://github.com/netlify/netlify-cms/issues/3671).

[erezrokah]

We update the image paths for new entries before saving the entry here:
https://github.com/netlify/netlify-cms/blob/81ce05f3fb43594036d4753f8414e6cb51340376/packages/netlify-cms-core/src/backend.ts#L1084

That handles the location of image files in the repo.
However, we don't fix it in the markdown content, hence the issue.

We'll need to somehow update the content based on the fields configuration. I can't think of an issue way to do it (yet).
However a possible workaround is to use the preSave event to fix the markdown content.