fix(smartplaylist): support isMissing/isPresent operators on ReplayGain fields (#5585)

* fix(smartplaylist): support isMissing/isPresent operators on ReplayGain fields

ReplayGain values are stored in dedicated nullable columns (rg_album_gain,
rg_album_peak, rg_track_gain, rg_track_peak) rather than in the media_file.tags
JSON blob. The isMissing/isPresent operators previously only supported tag and
role fields, causing two failure modes:

1. Using the documented alias names (replaygain_album_gain etc.) from PR #5256:
   these got registered as JSON tags from mappings.yaml, so isMissing queried
   json_tree(media_file.tags, '$.replaygain_album_gain') which is always empty
   -> the playlist matched ALL songs.

2. Using the canonical field names (rgalbumgain etc.): not a tag/role, so SQL
   generation returned an error. Because refreshSmartPlaylist deletes old tracks
   before regenerating, the abort left the playlist empty.

Fix: add Nullable bool to FieldInfo and mark the four ReplayGain fields. Add
static alias entries (replaygain_album_gain -> rgalbumgain etc.) with
Numeric+Nullable set; because AddTagNames skips names already in the field map,
these static entries take precedence over the mappings.yaml tag registration.
missingExpr now emits IS NULL / IS NOT NULL for nullable column fields instead
of the json_tree lookup.

Fixes #5584

* chore(smartplaylist): address code review feedback

- Simplify the isMissing/isPresent unsupported-field error message,
  removing the internal "nullable fields" jargon
- Standardize comments on mappings.yaml (the actual filename)
- Clarify the alias precedence comment in the LookupField test

model/criteria/fields.go

@@ -4,13 +4,14 @@ import "strings"
 
 // FieldInfo contains semantic metadata about a criteria field.
 type FieldInfo struct {
-	Alias   string // If set, this field is a backward-compat alias for another canonical name
-	IsTag   bool
-	IsRole  bool
-	Numeric bool
-	Boolean bool
+	Alias    string // If set, this field is a backward-compat alias for another canonical name
+	IsTag    bool
+	IsRole   bool
+	Numeric  bool
+	Boolean  bool
+	Nullable bool // If set, this column field can be NULL, so isMissing/isPresent are supported on it
 
-	tagAlias string // If set, a tag name from mappings.yml that resolves to this field
+	tagAlias string // If set, a tag name from mappings.yaml that resolves to this field
 	name     string // Canonical name, populated by LookupField from the map key
 }
 
@@ -80,15 +81,23 @@ var fieldMap = map[string]FieldInfo{
 	"mbz_recording_id":     {},
 	"mbz_release_track_id": {},
 	"mbz_release_group_id": {},
-	"rgalbumgain":          {Numeric: true},
-	"rgalbumpeak":          {Numeric: true},
-	"rgtrackgain":          {Numeric: true},
-	"rgtrackpeak":          {Numeric: true},
+	"rgalbumgain":          {Numeric: true, Nullable: true},
+	"rgalbumpeak":          {Numeric: true, Nullable: true},
+	"rgtrackgain":          {Numeric: true, Nullable: true},
+	"rgtrackpeak":          {Numeric: true, Nullable: true},
 	"library_id":           {Numeric: true},
 
 	// Backward compatibility: albumtype is an alias for the releasetype tag.
 	"albumtype": {Alias: "releasetype", IsTag: true},
 
+	// Backward compatibility: the replaygain_* tag names (as written in metadata and in the
+	// PR #5256 example) are aliases for the canonical rg* column fields. Without these, the tag
+	// names would be registered as empty tags from mappings.yaml and isMissing would always match.
+	"replaygain_album_gain": {Alias: "rgalbumgain", Numeric: true, Nullable: true},
+	"replaygain_album_peak": {Alias: "rgalbumpeak", Numeric: true, Nullable: true},
+	"replaygain_track_gain": {Alias: "rgtrackgain", Numeric: true, Nullable: true},
+	"replaygain_track_peak": {Alias: "rgtrackpeak", Numeric: true, Nullable: true},
+
 	// Pseudo-field for random sorting
 	"random": {},
 }
@@ -128,7 +137,7 @@ func AddRoles(roles []string) {
 	}
 }
 
-// AddTagNames adds tag names to the field map. This is used to add all tags mapped in the `mappings.yml`
+// AddTagNames adds tag names to the field map. This is used to add all tags mapped in the `mappings.yaml`
 // configuration file.
 func AddTagNames(tagNames []string) {
 	for _, tagName := range tagNames {

model/criteria/fields_test.go

@@ -53,5 +53,27 @@ var _ = Describe("fields", func() {
 			gomega.Expect(field.IsRole).To(gomega.BeTrue())
 		})
 
+		It("marks ReplayGain column fields as nullable", func() {
+			field, ok := LookupField("rgAlbumGain")
+
+			gomega.Expect(ok).To(gomega.BeTrue())
+			gomega.Expect(field.Name()).To(gomega.Equal("rgalbumgain"))
+			gomega.Expect(field.Nullable).To(gomega.BeTrue())
+			gomega.Expect(field.IsTag).To(gomega.BeFalse())
+		})
+
+		It("resolves replaygain_* tag names as aliases to nullable column fields", func() {
+			// AddTagNames skips names already in the field map, so the startup tag registration
+			// (from mappings.yaml) must not convert the pre-registered alias into a tag field.
+			AddTagNames([]string{"replaygain_album_gain"})
+
+			field, ok := LookupField("replaygain_album_gain")
+
+			gomega.Expect(ok).To(gomega.BeTrue())
+			gomega.Expect(field.Name()).To(gomega.Equal("rgalbumgain"))
+			gomega.Expect(field.Nullable).To(gomega.BeTrue())
+			gomega.Expect(field.IsTag).To(gomega.BeFalse())
+		})
+
 	})
 })