Centralise field type defs

[snejus]

Centralise Field Type Definitions

Summary

This PR eliminates duplicated field-to-type mappings across Album and Item model classes. Previously, each model class maintained its own _fields: dict[str, types.Type] — a verbose, hand-written dictionary that encoded both which fields exist and what type each one has. This duplication made it easy for the two definitions to drift out of sync.

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Architecture Change

Before: Each model class owned a full inline _fields dict mapping field names to type instances.

Album._fields = {"id": types.PRIMARY_ID, "artpath": types.NullPathType(), "added": types.DATE, ...}  # ~40 entries
Item._fields  = {"id": types.PRIMARY_ID, "path": types.PathType(), "album_id": types.FOREIGN_ID, ...}  # ~80 entries

After: Field names are declared as a set[str], and the type mapping is derived lazily from a single centralised TYPE_BY_FIELD registry in beets.dbcore.fields.

Album._field_names = {"id", "artpath", "added", ...}        # plain set of names
Item._field_names  = Album._field_names - {"artpath"} | {...} # extends Album's set

LibModel._fields (cached_classproperty) = {f: TYPE_BY_FIELD[f] for f in cls._field_names}

This creates a clean separation of concerns:

• What fields belong to a model → declared in the model (_field_names)
• What type a field has → declared once in dbcore.fields.TYPE_BY_FIELD

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Key Impacts

Area	Before	After
Type definitions	Inline per model	Single registry in dbcore.fields
Item._fields	Fully independent dict	Derived from Album._field_names
Album.item_keys	Hard-coded list of 40+ strings	_field_names - {"artpath", "id"}
_media_fields / _media_tag_fields	Intersected with _fields.keys()	Intersected with _field_names directly
Coverage measurement	--cov=beets --cov=beetsplug	--cov=. (fixes missing beetsplug/musicbrainz.py)

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Risk Areas to Review

• TYPE_BY_FIELD completeness: all field names in _field_names must have a corresponding entry in the registry — a missing key will raise at class definition time via cached_classproperty.
• Album.item_keys semantics: changed from list to set and is now derived automatically. Any code that relied on ordering or specific list membership should be checked.
• Item._field_names inheritance expression: Album._field_names - {"artpath"} | {...} — operator precedence here is (Album._field_names - {"artpath"}) | {...}, which is the intended behaviour, but worth a close read.

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.78%. Comparing base (82384da) to head (e2e8e80).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6550      +/-   ##
==========================================
+ Coverage   71.19%   71.78%   +0.59%     
==========================================
  Files         150      156       +6     
  Lines       19175    20176    +1001     
  Branches     3084     3214     +130     
==========================================
+ Hits        13651    14484     +833     
- Misses       4864     5006     +142     
- Partials      660      686      +26     

Files with missing lines	Coverage Δ
beets/library/fields.py	100.00% <100.00%> (ø)
beets/library/models.py	87.01% <100.00%> (+0.10%)	⬆️

... and 5 files with indirect coverage changes

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[Copilot]

Pull request overview

grug see PR want one place for field -> type mapping, so Album and Item no drift.

Changes:

• move Album/Item fixed field list to _field_names sets, and derive _fields from shared registry
• update media-field set logic to use _field_names, and derive Album.item_keys from field set math
• change coverage task to use --cov=.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
pyproject.toml	change pytest-cov source selection for coverage runs
beets/library/models.py	replace per-model _fields dicts with _field_names + derived _fields, adjust related logic

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.