feat(server): add percentage-based limits to smart playlists (#5144)

* feat(playlists): add percentage-based limits to smart playlists

Add a new `limitPercent` JSON field to Criteria that allows smart playlist
limits to be expressed as a percentage of matching tracks rather than a
fixed number. For example, a playlist matching 450 songs with a 10% limit
returns 45 songs, scaling dynamically as the library grows.

When `limitPercent` is set, refreshSmartPlaylist runs a COUNT query first
to determine the total matching tracks, then resolves the percentage to an
absolute LIMIT before executing the main query. The fixed `limit` field
takes precedence when both are set. Values are clamped to [0, 100] during
JSON unmarshaling.

No database migration is needed since rules are stored as a JSON string.

* fix(criteria): validate percentage limit range in IsPercentageLimit method

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(criteria): ensure idempotency of ToSql method for expressions

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

core/playlists/parse_nsp_test.go

@@ -122,6 +122,21 @@ var _ = Describe("parseNSP", func() {
 		Expect(pls.Name).To(Equal("Original"))
 	})
 
+	It("parses limitPercent from NSP", func() {
+		nsp := `{
+			"all": [{"is": {"loved": true}}],
+			"sort": "playCount",
+			"order": "desc",
+			"limitPercent": 25
+		}`
+		pls := &model.Playlist{}
+		err := s.parseNSP(ctx, pls, strings.NewReader(nsp))
+		Expect(err).ToNot(HaveOccurred())
+		Expect(pls.Rules).ToNot(BeNil())
+		Expect(pls.Rules.LimitPercent).To(Equal(25))
+		Expect(pls.Rules.Limit).To(Equal(0))
+	})
+
 	It("parses criteria with multiple rules", func() {
 		nsp := `{
 			"all": [

model/criteria/criteria.go

@@ -15,10 +15,38 @@ type Expression = squirrel.Sqlizer
 
 type Criteria struct {
 	Expression
-	Sort   string
-	Order  string
-	Limit  int
-	Offset int
+	Sort         string
+	Order        string
+	Limit        int
+	LimitPercent int
+	Offset       int
+}
+
+// EffectiveLimit resolves the effective limit for a query. If a fixed Limit is
+// set it takes precedence. Otherwise, if LimitPercent is set, the limit is
+// computed as a percentage of totalCount (minimum 1 when totalCount > 0).
+// Returns 0 when no limit applies.
+func (c Criteria) EffectiveLimit(totalCount int64) int {
+	if c.Limit > 0 {
+		return c.Limit
+	}
+	if c.LimitPercent > 0 && c.LimitPercent <= 100 {
+		if totalCount <= 0 {
+			return 0
+		}
+		result := int(totalCount) * c.LimitPercent / 100
+		if result < 1 {
+			return 1
+		}
+		return result
+	}
+	return 0
+}
+
+// IsPercentageLimit returns true when the criteria uses a valid percentage-based
+// limit (i.e. LimitPercent is in [1, 100] and no fixed Limit overrides it).
+func (c Criteria) IsPercentageLimit() bool {
+	return c.Limit == 0 && c.LimitPercent > 0 && c.LimitPercent <= 100
 }
 
 func (c Criteria) OrderBy() string {
@@ -95,6 +123,16 @@ func (c Criteria) ToSql() (sql string, args []any, err error) {
 	return c.Expression.ToSql()
 }
 
+// ExpressionJoins returns only the JOINs needed by the WHERE-clause expression,
+// excluding any JOINs required solely for sorting. This is useful for COUNT
+// queries where sort order is irrelevant.
+func (c Criteria) ExpressionJoins() JoinType {
+	if c.Expression == nil {
+		return JoinNone
+	}
+	return extractJoinTypes(c.Expression)
+}
+
 // RequiredJoins inspects the expression tree and Sort field to determine which
 // additional JOINs are needed when evaluating this criteria.
 func (c Criteria) RequiredJoins() JoinType {
@@ -128,17 +166,19 @@ func (c Criteria) ChildPlaylistIds() []string {
 
 func (c Criteria) MarshalJSON() ([]byte, error) {
 	aux := struct {
-		All    []Expression `json:"all,omitempty"`
-		Any    []Expression `json:"any,omitempty"`
-		Sort   string       `json:"sort,omitempty"`
-		Order  string       `json:"order,omitempty"`
-		Limit  int          `json:"limit,omitempty"`
-		Offset int          `json:"offset,omitempty"`
+		All          []Expression `json:"all,omitempty"`
+		Any          []Expression `json:"any,omitempty"`
+		Sort         string       `json:"sort,omitempty"`
+		Order        string       `json:"order,omitempty"`
+		Limit        int          `json:"limit,omitempty"`
+		LimitPercent int          `json:"limitPercent,omitempty"`
+		Offset       int          `json:"offset,omitempty"`
 	}{
-		Sort:   c.Sort,
-		Order:  c.Order,
-		Limit:  c.Limit,
-		Offset: c.Offset,
+		Sort:         c.Sort,
+		Order:        c.Order,
+		Limit:        c.Limit,
+		LimitPercent: c.LimitPercent,
+		Offset:       c.Offset,
 	}
 	switch rules := c.Expression.(type) {
 	case Any:
@@ -153,12 +193,13 @@ func (c Criteria) MarshalJSON() ([]byte, error) {
 
 func (c *Criteria) UnmarshalJSON(data []byte) error {
 	var aux struct {
-		All    unmarshalConjunctionType `json:"all"`
-		Any    unmarshalConjunctionType `json:"any"`
-		Sort   string                   `json:"sort"`
-		Order  string                   `json:"order"`
-		Limit  int                      `json:"limit"`
-		Offset int                      `json:"offset"`
+		All          unmarshalConjunctionType `json:"all"`
+		Any          unmarshalConjunctionType `json:"any"`
+		Sort         string                   `json:"sort"`
+		Order        string                   `json:"order"`
+		Limit        int                      `json:"limit"`
+		LimitPercent int                      `json:"limitPercent"`
+		Offset       int                      `json:"offset"`
 	}
 	if err := json.Unmarshal(data, &aux); err != nil {
 		return err
@@ -174,5 +215,15 @@ func (c *Criteria) UnmarshalJSON(data []byte) error {
 	c.Order = aux.Order
 	c.Limit = aux.Limit
 	c.Offset = aux.Offset
+
+	// Clamp LimitPercent to [0, 100]
+	if aux.LimitPercent < 0 {
+		log.Warn("limitPercent value out of range, clamping to 0", "value", aux.LimitPercent)
+		aux.LimitPercent = 0
+	} else if aux.LimitPercent > 100 {
+		log.Warn("limitPercent value out of range, clamping to 100", "value", aux.LimitPercent)
+		aux.LimitPercent = 100
+	}
+	c.LimitPercent = aux.LimitPercent
 	return nil
 }

model/criteria/criteria_test.go

@@ -181,6 +181,28 @@ var _ = Describe("Criteria", func() {
 		})
 	})
 
+	Describe("ExpressionJoins", func() {
+		It("excludes sort-only joins", func() {
+			c := Criteria{
+				Expression: All{
+					Contains{"title": "love"},
+				},
+				Sort: "albumRating",
+			}
+			gomega.Expect(c.ExpressionJoins()).To(gomega.Equal(JoinNone))
+			gomega.Expect(c.RequiredJoins().Has(JoinAlbumAnnotation)).To(gomega.BeTrue())
+		})
+
+		It("includes expression-based joins", func() {
+			c := Criteria{
+				Expression: All{
+					Gt{"albumRating": 3},
+				},
+			}
+			gomega.Expect(c.ExpressionJoins().Has(JoinAlbumAnnotation)).To(gomega.BeTrue())
+		})
+	})
+
 	Describe("RequiredJoins", func() {
 		It("returns JoinNone when no annotation fields are used", func() {
 			c := Criteria{
@@ -263,6 +285,126 @@ var _ = Describe("Criteria", func() {
 		})
 	})
 
+	Describe("LimitPercent", func() {
+		Describe("JSON round-trip", func() {
+			It("marshals and unmarshals limitPercent", func() {
+				goObj := Criteria{
+					Expression:   All{Contains{"title": "love"}},
+					Sort:         "title",
+					Order:        "asc",
+					LimitPercent: 10,
+				}
+				j, err := json.Marshal(goObj)
+				gomega.Expect(err).ToNot(gomega.HaveOccurred())
+				gomega.Expect(string(j)).To(gomega.ContainSubstring(`"limitPercent":10`))
+				gomega.Expect(string(j)).ToNot(gomega.ContainSubstring(`"limit"`))
+
+				var newObj Criteria
+				err = json.Unmarshal(j, &newObj)
+				gomega.Expect(err).ToNot(gomega.HaveOccurred())
+				gomega.Expect(newObj.LimitPercent).To(gomega.Equal(10))
+				gomega.Expect(newObj.Limit).To(gomega.Equal(0))
+			})
+
+			It("does not include limitPercent when zero", func() {
+				goObj := Criteria{
+					Expression: All{Contains{"title": "love"}},
+					Limit:      50,
+				}
+				j, err := json.Marshal(goObj)
+				gomega.Expect(err).ToNot(gomega.HaveOccurred())
+				gomega.Expect(string(j)).To(gomega.ContainSubstring(`"limit":50`))
+				gomega.Expect(string(j)).ToNot(gomega.ContainSubstring(`limitPercent`))
+			})
+
+			It("backward compatible: JSON with only limit still works", func() {
+				jsonStr := `{"all":[{"contains":{"title":"love"}}],"limit":20}`
+				var c Criteria
+				err := json.Unmarshal([]byte(jsonStr), &c)
+				gomega.Expect(err).ToNot(gomega.HaveOccurred())
+				gomega.Expect(c.Limit).To(gomega.Equal(20))
+				gomega.Expect(c.LimitPercent).To(gomega.Equal(0))
+			})
+		})
+
+		Describe("UnmarshalJSON clamping", func() {
+			It("clamps values above 100 to 100", func() {
+				jsonStr := `{"all":[{"contains":{"title":"love"}}],"limitPercent":150}`
+				var c Criteria
+				err := json.Unmarshal([]byte(jsonStr), &c)
+				gomega.Expect(err).ToNot(gomega.HaveOccurred())
+				gomega.Expect(c.LimitPercent).To(gomega.Equal(100))
+			})
+
+			It("clamps negative values to 0", func() {
+				jsonStr := `{"all":[{"contains":{"title":"love"}}],"limitPercent":-5}`
+				var c Criteria
+				err := json.Unmarshal([]byte(jsonStr), &c)
+				gomega.Expect(err).ToNot(gomega.HaveOccurred())
+				gomega.Expect(c.LimitPercent).To(gomega.Equal(0))
+			})
+		})
+
+		Describe("EffectiveLimit", func() {
+			It("returns fixed limit when Limit is set", func() {
+				c := Criteria{Limit: 50, LimitPercent: 10}
+				gomega.Expect(c.EffectiveLimit(1000)).To(gomega.Equal(50))
+			})
+
+			It("returns percentage-based limit", func() {
+				c := Criteria{LimitPercent: 10}
+				gomega.Expect(c.EffectiveLimit(450)).To(gomega.Equal(45))
+			})
+
+			It("returns minimum 1 when totalCount > 0 and percentage rounds to 0", func() {
+				c := Criteria{LimitPercent: 1}
+				gomega.Expect(c.EffectiveLimit(5)).To(gomega.Equal(1))
+			})
+
+			It("returns 0 when totalCount is 0", func() {
+				c := Criteria{LimitPercent: 10}
+				gomega.Expect(c.EffectiveLimit(0)).To(gomega.Equal(0))
+			})
+
+			It("returns 0 when no limit is set", func() {
+				c := Criteria{}
+				gomega.Expect(c.EffectiveLimit(1000)).To(gomega.Equal(0))
+			})
+
+			It("returns full count for 100%", func() {
+				c := Criteria{LimitPercent: 100}
+				gomega.Expect(c.EffectiveLimit(450)).To(gomega.Equal(450))
+			})
+
+			It("returns 1 for 1% of 50 items", func() {
+				c := Criteria{LimitPercent: 1}
+				gomega.Expect(c.EffectiveLimit(50)).To(gomega.Equal(1))
+			})
+		})
+
+		Describe("IsPercentageLimit", func() {
+			It("returns true when LimitPercent is set and Limit is 0", func() {
+				c := Criteria{LimitPercent: 10}
+				gomega.Expect(c.IsPercentageLimit()).To(gomega.BeTrue())
+			})
+
+			It("returns false when Limit is set", func() {
+				c := Criteria{Limit: 50, LimitPercent: 10}
+				gomega.Expect(c.IsPercentageLimit()).To(gomega.BeFalse())
+			})
+
+			It("returns false when neither is set", func() {
+				c := Criteria{}
+				gomega.Expect(c.IsPercentageLimit()).To(gomega.BeFalse())
+			})
+
+			It("returns false when LimitPercent is out of range", func() {
+				c := Criteria{LimitPercent: 150}
+				gomega.Expect(c.IsPercentageLimit()).To(gomega.BeFalse())
+			})
+		})
+	})
+
 	Context("with child playlists", func() {
 		var (
 			topLevelInPlaylistID     string

model/criteria/fields.go

@@ -122,27 +122,24 @@ func mapExpr(expr squirrel.Sqlizer, negate bool, exprFunc func(string, squirrel.
 		log.Fatal(fmt.Sprintf("expr is not a map-based operator: %T", expr))
 	}
 
-	// Extract into a generic map
+	// Extract the field name and value, then build a new map keyed by "value"
+	// for the inner condition. The original map is left untouched so that
+	// ToSql can be called multiple times without corruption.
 	var k string
-	m := make(map[string]any, rv.Len())
+	var v any
 	for _, key := range rv.MapKeys() {
-		// Save the key to build the expression, and use the provided keyName as the key
 		k = key.String()
-		m["value"] = rv.MapIndex(key).Interface()
+		v = rv.MapIndex(key).Interface()
 		break // only one key is expected (and supported)
 	}
 
-	// Clear the original map
-	for _, key := range rv.MapKeys() {
-		rv.SetMapIndex(key, reflect.Value{})
-	}
+	// Create a new map-based expression with "value" as the key, matching the
+	// column name inside json_tree subqueries.
+	newMap := reflect.MakeMap(rv.Type())
+	newMap.SetMapIndex(reflect.ValueOf("value"), reflect.ValueOf(v))
+	newExpr := newMap.Interface().(squirrel.Sqlizer)
 
-	// Write the updated map back into the original variable
-	for key, val := range m {
-		rv.SetMapIndex(reflect.ValueOf(key), reflect.ValueOf(val))
-	}
-
-	return exprFunc(k, expr, negate)
+	return exprFunc(k, newExpr, negate)
 }
 
 // mapTagExpr maps a normal field expression to a tag expression.

model/criteria/operators_test.go

@@ -178,6 +178,21 @@ var _ = Describe("Operators", func() {
 		})
 	})
 
+	DescribeTable("ToSql idempotency",
+		func(expr Expression) {
+			sql1, args1, err1 := expr.ToSql()
+			sql2, args2, err2 := expr.ToSql()
+
+			gomega.Expect(err1).ToNot(gomega.HaveOccurred())
+			gomega.Expect(err2).ToNot(gomega.HaveOccurred())
+			gomega.Expect(sql2).To(gomega.Equal(sql1))
+			gomega.Expect(args2).To(gomega.Equal(args1))
+		},
+		Entry("tag expression", Is{"genre": "Rock"}),
+		Entry("role expression", Contains{"artist": "Beatles"}),
+		Entry("nested criteria", Criteria{Expression: All{Is{"genre": "Rock"}, Contains{"artist": "Beatles"}}}),
+	)
+
 	DescribeTable("JSON Marshaling",
 		func(op Expression, jsonString string) {
 			obj := And{op}

persistence/playlist_repository.go

@@ -248,22 +248,36 @@ func (r *playlistRepository) refreshSmartPlaylist(pls *model.Playlist) bool {
 
 	// Conditionally join album/artist annotation tables only when referenced by criteria or sort
 	requiredJoins := rules.RequiredJoins()
-	if requiredJoins.Has(criteria.JoinAlbumAnnotation) {
-		sq = sq.LeftJoin("annotation AS album_annotation ON ("+
-			"album_annotation.item_id = media_file.album_id"+
-			" AND album_annotation.item_type = 'album'"+
-			" AND album_annotation.user_id = ?)", usr.ID)
-	}
-	if requiredJoins.Has(criteria.JoinArtistAnnotation) {
-		sq = sq.LeftJoin("annotation AS artist_annotation ON ("+
-			"artist_annotation.item_id = media_file.artist_id"+
-			" AND artist_annotation.item_type = 'artist'"+
-			" AND artist_annotation.user_id = ?)", usr.ID)
-	}
+	sq = r.addSmartPlaylistAnnotationJoins(sq, requiredJoins, usr.ID)
 
 	// Only include media files from libraries the user has access to
 	sq = r.applyLibraryFilter(sq, "media_file")
 
+	// Resolve percentage-based limit to an absolute number before applying criteria
+	if rules.IsPercentageLimit() {
+		// Use only expression-based joins for the COUNT query (sort joins are unnecessary)
+		exprJoins := rules.ExpressionJoins()
+		countSq := Select("count(*) as count").From("media_file").
+			LeftJoin("annotation on ("+
+				"annotation.item_id = media_file.id"+
+				" AND annotation.item_type = 'media_file'"+
+				" AND annotation.user_id = ?)", usr.ID)
+		countSq = r.addSmartPlaylistAnnotationJoins(countSq, exprJoins, usr.ID)
+		countSq = r.applyLibraryFilter(countSq, "media_file")
+		countSq = countSq.Where(rules)
+
+		var res struct{ Count int64 }
+		err = r.queryOne(countSq, &res)
+		if err != nil {
+			log.Error(r.ctx, "Error counting matching tracks for percentage limit", "playlist", pls.Name, "id", pls.ID, err)
+			return false
+		}
+		resolvedLimit := rules.EffectiveLimit(res.Count)
+		log.Debug(r.ctx, "Resolved percentage limit", "playlist", pls.Name, "percent", rules.LimitPercent, "totalMatching", res.Count, "resolvedLimit", resolvedLimit)
+		rules.Limit = resolvedLimit
+		rules.LimitPercent = 0
+	}
+
 	// Apply the criteria rules
 	sq = r.addCriteria(sq, rules)
 	insSql := Insert("playlist_tracks").Columns("id", "playlist_id", "media_file_id").Select(sq)
@@ -296,6 +310,22 @@ func (r *playlistRepository) refreshSmartPlaylist(pls *model.Playlist) bool {
 	return true
 }
 
+func (r *playlistRepository) addSmartPlaylistAnnotationJoins(sq SelectBuilder, joins criteria.JoinType, userID string) SelectBuilder {
+	if joins.Has(criteria.JoinAlbumAnnotation) {
+		sq = sq.LeftJoin("annotation AS album_annotation ON ("+
+			"album_annotation.item_id = media_file.album_id"+
+			" AND album_annotation.item_type = 'album'"+
+			" AND album_annotation.user_id = ?)", userID)
+	}
+	if joins.Has(criteria.JoinArtistAnnotation) {
+		sq = sq.LeftJoin("annotation AS artist_annotation ON ("+
+			"artist_annotation.item_id = media_file.artist_id"+
+			" AND artist_annotation.item_type = 'artist'"+
+			" AND artist_annotation.user_id = ?)", userID)
+	}
+	return sq
+}
+
 func (r *playlistRepository) addCriteria(sql SelectBuilder, c criteria.Criteria) SelectBuilder {
 	sql = sql.Where(c)
 	if c.Limit > 0 {