Add support for setting custom identifier field

[ascorbic]

- Summary

Sometimes title doesn't make sense as the identifier for a collection. For example I have a JSON collection called clients, which has a name field which I want to use for the slug. This PR adds support for an identifier_field setting for collections, which lets the user choose a different identifier field.

Closes #1802, #1700.

- Test plan

Try using the following collection:

- label: "Blog"
  name: "blog"
  folder: "_posts/blog"
  create: true
  identifier_field: name
  fields:
    - {label: "Name", name: "name", widget: "string"}
    - {label: "Publish Date", name: "date", widget: "datetime"}
    - {label: "Featured Image", name: "thumbnail", widget: "image"}
    - {label: "Body", name: "body", widget: "markdown"}

- Description for the changelog

Allow identifier fields other than title or path.

- A picture of a cute animal (not mandatory but encouraged)

[verythorough]

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[verythorough]

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[erquhart]

Thanks for this!

Just a heads up: anything that touches the slug requires thorough examination. Your changes are quite simple, it's the impact to seemingly unrelated parts of the project that we need to be sure about. Apologies this was overlooked for so long! We'll get it reviewed soon.

[ascorbic]

Great. Thanks for the update.

[radusuciu]

Also would like to see this merged. This + support for custom slug synthesis from fields is the dream.

[erquhart]

@radusuciu you can reference fields by name as placeholders in the slug field for a collection, fyi.

[erquhart]

Rebased, looks good to me - thanks @ascorbic!

Also tested locally.

[0x80]

I started using Netlify CMS just today, and ran into this issue straight away 😅

In my index.html I'm getting the script from https://unpkg.com/netlify-cms@^2.0.0/dist/netlify-cms.js. Do I need to do anything special to get this recent update?

Right now I'm stuck with this error:

'collections[0]' must have a field that is a valid entry identifier

And my collection config is this:

collections:
  - name: artists
    label: Artists
    label_singular: Artist
    folder: data/artists
    identifier_field: name
    create: true
    fields:
      - { label: 'Name', name: 'name', widget: 'string', description: 'The artist name' }
      - { label: 'Company Name', name: 'company_name', widget: 'string', description: 'The company name (optional)', required: false }
      - { label: 'Image', name: 'image', widget: 'image', description: A profile image }
      - { label: 'Short', name: 'short', widget: 'string', description: 'A short one-liner description' }
      - { label: 'Link', name: 'link', widget: 'string', description: 'A link to the artists website or other external resource' }
      - { label: 'Building Nr', name: 'building', widget: 'select', options: [1, 2, 3] }
      - { label: 'House Nr', name: 'house', widget: number }

[0x80]

Ah I see. This is merged but not published yet as a new version, right?

[erquhart]

Yep, coming soon.