funkwhale/docs/specs/multi-artist/mb-content.md

Multi-artist support for MusicBrainz-tagged content

Terminology

The following terminology is used throughout this spec:

Join phrase

The token used to split a list of artists and represent their role in content

Release

An album or single. In Funkwhale a release is represented by the Album model

Recording

A single track as part of an album or single. In Funkwhale, a recording is represented by the Track model

Proposed solution

To support multi-artist content, a new ArtistCredit model should be added to house contributing artist information, including links to Artist models and details about credited names and join phrases. The ArtistCredit model should link to Album and Track entries using a ForeignKey relationship to allow multiple artist_credit entries on each Album and Track. The existing artist field in Album and Track models is deprecated in favor of the new artist_credit field.

A fetch_credit strategy should be used to fetch artist credit directly from MusicBrainz's entry for releases and recordings.

Feature behavior

The workflow for ArtistCredit population goes as follows:

flowchart TD
    upload([The user uploads content to Funkwhale])
    upload --> read(Funkwhale reads the file tags of each release/recording)
    read --> mb-tagged{Does the content have a MusicBrainz ID?}
    mb-tagged -->|yes| query(Funkwhale queries the MusicBrainz API to\nretrieve artist information for each\nunique mbid)
    query --> create(Funkwhale fetches artist information\nand creates Artist entries for any\nartists not present in the database)
    create --> model(Funkwhale stores the artist_credit information\nfor each release/recording)
    model --> display([Funkwhale displays the credit for each release/recording\nin the web app/API representation])

Backend

The backend is responsible for

1. Tokenizing artist tags when a MusicBrainz ID isn't present in the file metadata and creating Artist entries for new artists
2. Querying the MusicBrainz API when a MusicBrainz ID is present and creating Artist entries for new artists
3. Creating ArtistCredit entries for each unique combination of artist_id, credit, and joinphrase
4. Adding the ArtistCredit links to Track and Album entries

The artist_credit entries must be linked sequentially to preserve ordering. For each entry, Funkwhale should query for an existing ArtistCredit entry and return it if found, or create a new one and return the new entry. This ensures that artist credit information is presented in the same order as in the tags.

Data model

A new ArtistCredit model needs to hold the contributing artist information. This model contains the following fields:

• The Funkwhale artist_id of the artist associated with the release or recording
• The credit representing the credited name of the artist as given for the release or recording
• The joinphrase associated with the artist (for example: " feat. ", " & ")

classDiagram
    direction LR
    class Album {
        String title
        ArtistCredit artist_credit
        Date release_date
        String release_group_id
        Attachment attachment_cover
        String type
        Actor attributed_to
        TaggedItem tagged_items
        Fetch fetches
        Content description
        List api_includes
        String api
        String federation_namespace
        MusicBrainz musicbrainz_model
        String musicbrainz_mapping
    }
    class Track {
        UUID mbid
        String title
        ArtistCredit artist_credit
        Integer disc_number
        Integer position
        Album album
        License license
        Actor attributed_to
        String copyright
        Attachment attachment_cover
        Integer downloads_count
        String federation_namespace
        String musicbrainz_model
        String api
        List api_includes
        MusicBrainz musicbrainz_mapping
        String import_hooks
        QuerySet objects
        Fetch fetches
    }
    class Artist {
        String name
        String federation_namespace
        String musicbrainz_model
        Object musicbrainz_mapping
        Actor attributed_to
        TaggedItem tagged_items
        Fetch fetches
        Content description
        Attachment attachment_cover
        String content_category
        Date modification_date
        String api
        QuerySet objects
    }
    class ArtistCredit {
        Artist artist_id
        String credit
        String joinphrase
    }
    Track "1" --> Album : album
    Track "1..*" --> ArtistCredit : artist_credit
    Album "1..*" --> ArtistCredit : artist_credit
    ArtistCredit "1" --> Artist : Artist

Workflow

On the backend, the workflow for fetching ArtistCredit information is as follows:

sequenceDiagram
    ArtistCredit ->> fetch_credit: Fetch credit from MusicBrainz
    fetch_credit -->> ArtistCredit: Return credit information from MusicBrainz

If MusicBrainz tags are present in the file metadata, the fetch_credit strategy should always be preferred to ensure that content aligns with MusicBrainz's tagging.

To maintain compatibility with MusicBrainz and ensure that Funkwhale can parse multi-artist releases, the MusicBrainz API should be used to fetch contributor information relating to the release.

The /release and /recording endpoints return artist-credit information when called with a inc=artists parameter. This information can be parsed to fetch relevant information:

• The joinphrase
• The MusicBrainz id of the artist
• The artist's name

https://musicbrainz.org/ws/2/release/ef140c88-8bf1-4e50-9555-5c1d1ed5865c?fmt=json&inc=artists

{
  "asin": null,
  "cover-art-archive": {
    "darkened": false,
    "artwork": true,
    "front": true,
    "back": false,
    "count": 1
  },
  "release-events": [
    {
      "area": {
        "id": "525d4e18-3d00-31b9-a58b-a146a916de8f",
        "type": null,
        "name": "[Worldwide]",
        "disambiguation": "",
        "sort-name": "[Worldwide]",
        "iso-3166-1-codes": ["XW"],
        "type-id": null
      },
      "date": "2012-11-15"
    }
  ],
  "quality": "normal",
  "barcode": "",
  "date": "2012-11-15",
  "status-id": "4e304316-386d-3409-af2e-78857eec5cfe",
  "status": "Official",
  "title": "One Christmas at a Time",
  "disambiguation": "",
  "text-representation": {
    "script": "Latn",
    "language": "eng"
  },
  "packaging-id": "119eba76-b343-3e02-a292-f0f00644bb9b",
  "packaging": "None",
  "country": "XW",
  "id": "ef140c88-8bf1-4e50-9555-5c1d1ed5865c",
  "artist-credit": [
    {
      "joinphrase": " & ",
      "name": "Jonathan Coulton",
      "artist": {
        "type-id": "b6e035f4-3ce9-331c-97df-83397230b0df",
        "name": "Jonathan Coulton",
        "id": "d8df7087-06d5-4545-9024-831bb8558ad1",
        "sort-name": "Coulton, Jonathan",
        "disambiguation": "",
        "type": "Person"
      }
    },
    {
      "artist": {
        "type": "Person",
        "disambiguation": "",
        "id": "7b5b87d3-f3ee-4b5d-b111-1f2e87f87124",
        "sort-name": "Roderick, John",
        "name": "John Roderick",
        "type-id": "b6e035f4-3ce9-331c-97df-83397230b0df"
      },
      "name": "John Roderick",
      "joinphrase": ""
    }
  ]
}

To catalogue multi-artist content, Funkwhale needs to:

1. Fetch information for each contributing artist and create Artist objects for any that don't exist
2. Create ArtistCredit objects associated with each release and recording for each unique combination of artist_id, credit, and joinphrase
3. If a matching ArtistCredit entry is found, Funkwhale should link to this

sequenceDiagram
    Funkwhale ->> MusicBrainz: /release/{mbid}?fmt=json&inc=artists
    MusicBrainz -->> Funkwhale: Album and ArtistCredit information
    Funkwhale ->> MusicBrainz: /recording/{mbid}?fmt=json&inc=artists
    MusicBrainz -->> Funkwhale: Track and ArtistCredit information
    loop For each unique artist mbid
        Funkwhale ->> MusicBrainz: /artist/{mbid}?fmt=json&inc=aliases
        MusicBrainz -->> Funkwhale: Artist information
    end

Given the above example, Funkwhale would create the following ArtistCredit object:

id	artist_id	credit	joinphrase
1	1	Jonathan Coulton	&
2	2	John Roderick

API behavior

The Funkwhale API needs to return artist credit information in a way that is easily consumed by a client.

Endpoints should include a artist filter that allows a client to return results for which artists are credited. This filter should take a list of IDs.

To return any albums where the artist is listed in the artist_credit field, you can filter by the artist_id field using the artist filter:

https://open.audio/api/v2/albums?artist=6451,6452

The credit field of the artist_credit object must also be searchable using a standard query:

https://tanukitunes.com/api/v2/albums?q=jonathan+coulton

Migration

To ensure all content has artist_credit information, an initial migration should copy the current artist information into an ArtistCredit object for each Album and Track with the following mapping:

• artist_id = Artist id
• credit = Artist name
• joinphrase = ""

Each album/track must have at least one artist_credit entry listed against it.

Frontend

Data needs to be clearly presented in the web app to ensure that users can easily see which artists collaborated on a work and discover more of their work.

Representation in releases and tracks

The frontend should use the artist_credit field to populate artist links on releases and tracks. Credited names and join phrases must be preserved to line up with the artists' intent.

The frontend should format the results as follows for each artist:

<span>
  <a href="{funkwhale_url}/library/artists/{artist_id}">{credit}</a>{joinphrase}
</span>

For example, given the following artist_credit response:

[
  {
    "artist_id": 25,
    "credit": "Jonathan Coulton",
    "joinphrase": " & "
  },
  {
    "artist_id": 395,
    "credit": "John Roderick",
    "joinphrase": ""
  }
]

The frontend would render the following:

<span>
  <a href="https://open.audio/library/artists/25">Jonathan Coulton</a> &
  <a href="https://open.audio/library/artists/395">John Roderick</a>
</span>

"Also appears in" section

To keep artist discographies properly catalogued, only albums attributed to the artist, not albums with tracks featuring the artist, should appear in the "Albums by" section of the artist page.

Releases to which the artist has contributed should be separated into a new section labeled "Also appears in". This ensures that it is easy to discover content to which an artist has contributed while also prioritizing their own content on the artist page.

"Compilations" page

The "Various Artists" artist is a special artist used in MusicBrainz to catalogue compilation content not attributed primarily to a specific artist or artists. This artist is widely used in Funkwhale as the Album Artist tag for compilation content. To preserve this content and make compilation content easier to find, we should create a special "Compilations" page to house compilation content.

Documentation

The following needs to be documented:

• The artist splitting mechanism. Meaningful examples should be provided to help users tag their content
• The environment setting to override the join phrase list
• Any tasks or script functions which are created to automate the update of historical content

Availability

• Admin panel
• App frontend
• CLI

Responsible parties

• The backend developers in the Development group are responsible for:
  • Creating the new ArtistCredit model
  • Updating the Album and Track models to link to ArtistCredit entries
  • Creating the strategies for populating ArtistCredit during the upload process
  • Updating the API/creating new endpoints
  • Creating the database migration to add the new field and that artist information is copied over
• The frontend developers in the Development group are responsible for:
  • Updating the artist representations in the Album and Track pages to use the new artist_credit field in the API response
  • Creating the new "Also appears in" section for the artist discography pages
• The Design group is responsible for:
  • Providing designs for how multi-artist content should be rendered responsively
  • Providing designs for the new "Compilations" page
  • Providing designs for the updated Artist page
• The Documentation group is responsible for:
  • Documenting the new feature behavior
  • Documenting the new environment settings
  • Providing docstrings for API development

Open questions

• What should be used as the default join words list?
• To maintain compatibility with API v1, should we continue to populate the artist tag on Track and Album entries with the first credited artist?

Minimum viable product

The MVP for this product is an implementation of the backend behavior. It should:

1. Set up the new models
2. Create the database migration

Next steps

1. Create a manage.py script that enables admins to update entries in the database
2. Create a task that updates files imported in-place