diff --git a/README.md b/README.md
index e69de29..d57b0cd 100644
--- a/README.md
+++ b/README.md
@@ -0,0 +1,3 @@
+# 🍥 Nigiri
+
+Delicious, hand-crafted in-memory database with JSON persistence.
\ No newline at end of file
diff --git a/collection.go b/collection.go
new file mode 100644
index 0000000..da8e3b2
--- /dev/null
+++ b/collection.go
@@ -0,0 +1,195 @@
+package nigiri
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"sync"
+)
+
+// StoreManager interface for stores that can load/save
+type StoreManager interface {
+	LoadData(path string) error
+	SaveData(path string) error
+	Clear()
+}
+
+// Collection manages multiple stores in a directory
+type Collection struct {
+	baseDir string
+	stores  map[string]StoreManager
+	mu      sync.RWMutex
+}
+
+// NewCollection creates a new collection in the specified directory
+func NewCollection(baseDir string) *Collection {
+	return &Collection{
+		baseDir: baseDir,
+		stores:  make(map[string]StoreManager),
+	}
+}
+
+// Register adds a store to the collection with a given name
+func (c *Collection) Register(name string, store StoreManager) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.stores[name] = store
+}
+
+// Unregister removes a store from the collection
+func (c *Collection) Unregister(name string) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	delete(c.stores, name)
+}
+
+// GetStore retrieves a registered store by name
+func (c *Collection) GetStore(name string) (StoreManager, bool) {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+	store, exists := c.stores[name]
+	return store, exists
+}
+
+// LoadAll loads all registered stores from their JSON files
+func (c *Collection) LoadAll() error {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	if err := os.MkdirAll(c.baseDir, 0755); err != nil {
+		return fmt.Errorf("failed to create base directory: %w", err)
+	}
+
+	var firstError error
+	for name, store := range c.stores {
+		path := filepath.Join(c.baseDir, name+".json")
+		if err := store.LoadData(path); err != nil && firstError == nil {
+			firstError = fmt.Errorf("failed to load %s: %w", name, err)
+		}
+	}
+
+	return firstError
+}
+
+// SaveAll saves all registered stores to their JSON files
+func (c *Collection) SaveAll() error {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	if err := os.MkdirAll(c.baseDir, 0755); err != nil {
+		return fmt.Errorf("failed to create base directory: %w", err)
+	}
+
+	var firstError error
+	for name, store := range c.stores {
+		path := filepath.Join(c.baseDir, name+".json")
+		if err := store.SaveData(path); err != nil && firstError == nil {
+			firstError = fmt.Errorf("failed to save %s: %w", name, err)
+		}
+	}
+
+	return firstError
+}
+
+// LoadStore loads a specific store by name
+func (c *Collection) LoadStore(name string) error {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	store, exists := c.stores[name]
+	if !exists {
+		return fmt.Errorf("store %s not registered", name)
+	}
+
+	path := filepath.Join(c.baseDir, name+".json")
+	return store.LoadData(path)
+}
+
+// SaveStore saves a specific store by name
+func (c *Collection) SaveStore(name string) error {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	store, exists := c.stores[name]
+	if !exists {
+		return fmt.Errorf("store %s not registered", name)
+	}
+
+	if err := os.MkdirAll(c.baseDir, 0755); err != nil {
+		return fmt.Errorf("failed to create base directory: %w", err)
+	}
+
+	path := filepath.Join(c.baseDir, name+".json")
+	return store.SaveData(path)
+}
+
+// ClearAll clears all registered stores
+func (c *Collection) ClearAll() {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	for _, store := range c.stores {
+		store.Clear()
+	}
+}
+
+// ClearStore clears a specific store by name
+func (c *Collection) ClearStore(name string) error {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	store, exists := c.stores[name]
+	if !exists {
+		return fmt.Errorf("store %s not registered", name)
+	}
+
+	store.Clear()
+	return nil
+}
+
+// ListStores returns names of all registered stores
+func (c *Collection) ListStores() []string {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	names := make([]string, 0, len(c.stores))
+	for name := range c.stores {
+		names = append(names, name)
+	}
+	return names
+}
+
+// GetPath returns the file path for a store name
+func (c *Collection) GetPath(name string) string {
+	return filepath.Join(c.baseDir, name+".json")
+}
+
+// StoreExists checks if a store file exists on disk
+func (c *Collection) StoreExists(name string) bool {
+	path := c.GetPath(name)
+	_, err := os.Stat(path)
+	return err == nil
+}
+
+// RemoveStoreFile removes the JSON file for a store
+func (c *Collection) RemoveStoreFile(name string) error {
+	path := c.GetPath(name)
+	err := os.Remove(path)
+	if os.IsNotExist(err) {
+		return nil // Already doesn't exist
+	}
+	return err
+}
+
+// Helper function to create typed store accessor
+func GetTypedStore[T any](collection *Collection, name string) (*BaseStore[T], bool) {
+	store, exists := collection.GetStore(name)
+	if !exists {
+		return nil, false
+	}
+
+	if typedStore, ok := store.(*BaseStore[T]); ok {
+		return typedStore, true
+	}
+	return nil, false
+}
diff --git a/constraints.go b/constraints.go
new file mode 100644
index 0000000..f761f8a
--- /dev/null
+++ b/constraints.go
@@ -0,0 +1,119 @@
+package nigiri
+
+import (
+	"reflect"
+	"strings"
+)
+
+// Constraint types
+type ConstraintType string
+
+const (
+	ConstraintUnique   ConstraintType = "unique"
+	ConstraintForeign  ConstraintType = "fkey"
+	ConstraintRequired ConstraintType = "required"
+	ConstraintIndex    ConstraintType = "index"
+)
+
+type FieldConstraint struct {
+	Type      ConstraintType
+	Field     string
+	Target    string // for foreign keys: "table.field"
+	IndexName string // for custom index names
+}
+
+type SchemaInfo struct {
+	Fields      map[string]reflect.Type
+	Constraints map[string][]FieldConstraint
+	Indices     map[string]string // field -> index name
+}
+
+// ParseSchema extracts constraints from struct tags
+func ParseSchema[T any]() *SchemaInfo {
+	var zero T
+	t := reflect.TypeOf(zero)
+	if t.Kind() == reflect.Ptr {
+		t = t.Elem()
+	}
+
+	schema := &SchemaInfo{
+		Fields:      make(map[string]reflect.Type),
+		Constraints: make(map[string][]FieldConstraint),
+		Indices:     make(map[string]string),
+	}
+
+	for i := 0; i < t.NumField(); i++ {
+		field := t.Field(i)
+		fieldName := field.Name
+		schema.Fields[fieldName] = field.Type
+
+		dbTag := field.Tag.Get("db")
+		if dbTag == "" {
+			continue
+		}
+
+		constraints := parseDBTag(fieldName, dbTag)
+		if len(constraints) > 0 {
+			schema.Constraints[fieldName] = constraints
+		}
+
+		// Auto-create indices for unique and indexed fields
+		for _, constraint := range constraints {
+			if constraint.Type == ConstraintUnique || constraint.Type == ConstraintIndex {
+				indexName := constraint.IndexName
+				if indexName == "" {
+					indexName = fieldName + "_idx"
+				}
+				schema.Indices[fieldName] = indexName
+			}
+		}
+	}
+
+	return schema
+}
+
+func parseDBTag(fieldName, tag string) []FieldConstraint {
+	var constraints []FieldConstraint
+	parts := strings.Split(tag, ",")
+
+	for _, part := range parts {
+		part = strings.TrimSpace(part)
+		if part == "" {
+			continue
+		}
+
+		switch {
+		case part == "unique":
+			constraints = append(constraints, FieldConstraint{
+				Type:  ConstraintUnique,
+				Field: fieldName,
+			})
+		case part == "required":
+			constraints = append(constraints, FieldConstraint{
+				Type:  ConstraintRequired,
+				Field: fieldName,
+			})
+		case part == "index":
+			constraints = append(constraints, FieldConstraint{
+				Type:  ConstraintIndex,
+				Field: fieldName,
+			})
+		case strings.HasPrefix(part, "index:"):
+			indexName := strings.TrimPrefix(part, "index:")
+			constraints = append(constraints, FieldConstraint{
+				Type:      ConstraintIndex,
+				Field:     fieldName,
+				IndexName: indexName,
+			})
+		case strings.HasPrefix(part, "fkey:"):
+			target := strings.TrimPrefix(part, "fkey:")
+			constraints = append(constraints, FieldConstraint{
+				Type:   ConstraintForeign,
+				Field:  fieldName,
+				Target: target,
+			})
+		}
+	}
+
+	return constraints
+}
diff --git a/persistence.go b/persistence.go
new file mode 100644
index 0000000..e1adf09
--- /dev/null
+++ b/persistence.go
@@ -0,0 +1,137 @@
+package nigiri
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"reflect"
+	"sort"
+)
+
+// LoadFromJSON loads items from JSON using reflection
+func (bs *BaseStore[T]) LoadFromJSON(filename string) error {
+	bs.mu.Lock()
+	defer bs.mu.Unlock()
+
+	data, err := os.ReadFile(filename)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil
+		}
+		return fmt.Errorf("failed to read JSON: %w", err)
+	}
+
+	if len(data) == 0 {
+		return nil
+	}
+
+	// Create slice of pointers to T
+	sliceType := reflect.SliceOf(reflect.PointerTo(bs.itemType))
+	slicePtr := reflect.New(sliceType)
+
+	if err := json.Unmarshal(data, slicePtr.Interface()); err != nil {
+		return fmt.Errorf("failed to unmarshal JSON: %w", err)
+	}
+
+	// Clear existing data
+	bs.items = make(map[int]*T)
+	bs.maxID = 0
+
+	// Clear unique indices
+	for fieldName := range bs.uniqueIndices {
+		bs.uniqueIndices[fieldName] = make(map[any]int)
+	}
+
+	// Extract items using reflection
+	slice := slicePtr.Elem()
+	for i := 0; i < slice.Len(); i++ {
+		item := slice.Index(i).Interface().(*T)
+
+		// Get ID using reflection
+		itemValue := reflect.ValueOf(item).Elem()
+		idField := itemValue.FieldByName("ID")
+		if !idField.IsValid() {
+			return fmt.Errorf("item type must have an ID field")
+		}
+
+		id := int(idField.Int())
+		bs.items[id] = item
+		if id > bs.maxID {
+			bs.maxID = id
+		}
+
+		// Update unique indices
+		bs.updateUniqueIndices(id, item, true)
+	}
+
+	return nil
+}
+
+// SaveToJSON saves items to JSON atomically with consistent ID ordering
+func (bs *BaseStore[T]) SaveToJSON(filename string) error {
+	bs.mu.RLock()
+	defer bs.mu.RUnlock()
+
+	// Get sorted IDs for consistent ordering
+	ids := make([]int, 0, len(bs.items))
+	for id := range bs.items {
+		ids = append(ids, id)
+	}
+	sort.Ints(ids)
+
+	// Build items slice in ID order
+	items := make([]*T, 0, len(bs.items))
+	for _, id := range ids {
+		items = append(items, bs.items[id])
+	}
+
+	data, err := json.MarshalIndent(items, "", "\t")
+	if err != nil {
+		return fmt.Errorf("failed to marshal to JSON: %w", err)
+	}
+
+	// Atomic write
+	tempFile := filename + ".tmp"
+	if err := os.WriteFile(tempFile, data, 0644); err != nil {
+		return fmt.Errorf("failed to write temp JSON: %w", err)
+	}
+
+	if err := os.Rename(tempFile, filename); err != nil {
+		os.Remove(tempFile)
+		return fmt.Errorf("failed to rename temp JSON: %w", err)
+	}
+
+	return nil
+}
+
+// LoadData loads from JSON file or starts empty
+func (bs *BaseStore[T]) LoadData(dataPath string) error {
+	if err := bs.LoadFromJSON(dataPath); err != nil {
+		if os.IsNotExist(err) {
+			fmt.Println("No existing data found, starting with empty store")
+			return nil
+		}
+		return fmt.Errorf("failed to load from JSON: %w", err)
+	}
+
+	fmt.Printf("Loaded %d items from %s\n", len(bs.items), dataPath)
+	bs.RebuildIndices() // Rebuild indices after loading
+	return nil
+}
+
+// SaveData saves to JSON file
+func (bs *BaseStore[T]) SaveData(dataPath string) error {
+	// Ensure directory exists
+	dataDir := filepath.Dir(dataPath)
+	if err := os.MkdirAll(dataDir, 0755); err != nil {
+		return fmt.Errorf("failed to create data directory: %w", err)
+	}
+
+	if err := bs.SaveToJSON(dataPath); err != nil {
+		return fmt.Errorf("failed to save to JSON: %w", err)
+	}
+
+	fmt.Printf("Saved %d items to %s\n", len(bs.items), dataPath)
+	return nil
+}
diff --git a/store.go b/store.go
new file mode 100644
index 0000000..dab3e8f
--- /dev/null
+++ b/store.go
@@ -0,0 +1,533 @@
+package nigiri
+
+import (
+	"fmt"
+	"maps"
+	"reflect"
+	"sort"
+	"strings"
+	"sync"
+)
+
+// Validatable interface for entities that can validate themselves
+type Validatable interface {
+	Validate() error
+}
+
+// IndexBuilder function type for building custom indices
+type IndexBuilder[T any] func(allItems map[int]*T) any
+
+// BaseStore provides generic storage with index management and constraints
+type BaseStore[T any] struct {
+	items         map[int]*T
+	maxID         int
+	mu            sync.RWMutex
+	itemType      reflect.Type
+	indices       map[string]any
+	indexBuilders map[string]IndexBuilder[T]
+	schema        *SchemaInfo
+	uniqueIndices map[string]map[any]int // field -> value -> id
+}
+
+// NewBaseStore creates a new base store for type T with schema parsing
+func NewBaseStore[T any]() *BaseStore[T] {
+	var zero T
+	schema := ParseSchema[T]()
+
+	store := &BaseStore[T]{
+		items:         make(map[int]*T),
+		maxID:         0,
+		itemType:      reflect.TypeOf(zero),
+		indices:       make(map[string]any),
+		indexBuilders: make(map[string]IndexBuilder[T]),
+		schema:        schema,
+		uniqueIndices: make(map[string]map[any]int),
+	}
+
+	// Initialize unique indices
+	for fieldName, constraints := range schema.Constraints {
+		for _, constraint := range constraints {
+			if constraint.Type == ConstraintUnique {
+				store.uniqueIndices[fieldName] = make(map[any]int)
+			}
+		}
+	}
+
+	// Auto-register indices for indexed fields
+	store.registerSchemaIndices()
+
+	return store
+}
+
+func (bs *BaseStore[T]) registerSchemaIndices() {
+	for fieldName, indexName := range bs.schema.Indices {
+		bs.RegisterIndex(indexName, BuildFieldLookupIndex[T](fieldName))
+	}
+}
+
+// ValidateConstraints checks all constraints for an item
+func (bs *BaseStore[T]) ValidateConstraints(id int, item *T) error {
+	itemValue := reflect.ValueOf(item).Elem()
+
+	for fieldName, constraints := range bs.schema.Constraints {
+		fieldValue := itemValue.FieldByName(fieldName)
+		if !fieldValue.IsValid() {
+			continue
+		}
+
+		for _, constraint := range constraints {
+			switch constraint.Type {
+			case ConstraintRequired:
+				if isZeroValue(fieldValue) {
+					return fmt.Errorf("field %s is required", fieldName)
+				}
+			case ConstraintUnique:
+				value := fieldValue.Interface()
+				if existingID, exists := bs.uniqueIndices[fieldName][value]; exists && existingID != id {
+					return fmt.Errorf("field %s value %v already exists", fieldName, value)
+				}
+			}
+		}
+	}
+
+	return nil
+}
+
+func isZeroValue(v reflect.Value) bool {
+	return v.Interface() == reflect.Zero(v.Type()).Interface()
+}
+
+func (bs *BaseStore[T]) updateUniqueIndices(id int, item *T, add bool) {
+	itemValue := reflect.ValueOf(item).Elem()
+
+	for fieldName := range bs.uniqueIndices {
+		fieldValue := itemValue.FieldByName(fieldName)
+		if !fieldValue.IsValid() {
+			continue
+		}
+
+		value := fieldValue.Interface()
+		if add {
+			bs.uniqueIndices[fieldName][value] = id
+		} else {
+			delete(bs.uniqueIndices[fieldName], value)
+		}
+	}
+}
+
+// RegisterIndex registers an index builder function
+func (bs *BaseStore[T]) RegisterIndex(name string, builder IndexBuilder[T]) {
+	bs.mu.Lock()
+	defer bs.mu.Unlock()
+	bs.indexBuilders[name] = builder
+}
+
+// GetIndex retrieves a named index
+func (bs *BaseStore[T]) GetIndex(name string) (any, bool) {
+	bs.mu.RLock()
+	defer bs.mu.RUnlock()
+	index, exists := bs.indices[name]
+	return index, exists
+}
+
+// RebuildIndices rebuilds all registered indices
+func (bs *BaseStore[T]) RebuildIndices() {
+	bs.mu.Lock()
+	defer bs.mu.Unlock()
+	bs.rebuildIndicesUnsafe()
+}
+
+func (bs *BaseStore[T]) rebuildIndicesUnsafe() {
+	allItems := make(map[int]*T, len(bs.items))
+	maps.Copy(allItems, bs.items)
+
+	for name, builder := range bs.indexBuilders {
+		bs.indices[name] = builder(allItems)
+	}
+}
+
+// AddWithValidation adds item with constraint validation and index rebuild
+func (bs *BaseStore[T]) AddWithValidation(id int, item *T) error {
+	bs.mu.Lock()
+	defer bs.mu.Unlock()
+
+	// Validate constraints
+	if err := bs.ValidateConstraints(id, item); err != nil {
+		return err
+	}
+
+	// Custom validation
+	if validatable, ok := any(item).(Validatable); ok {
+		if err := validatable.Validate(); err != nil {
+			return err
+		}
+	}
+
+	// Update unique indices
+	bs.updateUniqueIndices(id, item, true)
+
+	bs.items[id] = item
+	if id > bs.maxID {
+		bs.maxID = id
+	}
+
+	bs.rebuildIndicesUnsafe()
+	return nil
+}
+
+// AddWithRebuild adds item with validation and index rebuild
+func (bs *BaseStore[T]) AddWithRebuild(id int, item *T) error {
+	return bs.AddWithValidation(id, item)
+}
+
+// RemoveWithValidation removes item and updates constraints
+func (bs *BaseStore[T]) RemoveWithValidation(id int) {
+	bs.mu.Lock()
+	defer bs.mu.Unlock()
+
+	if item, exists := bs.items[id]; exists {
+		bs.updateUniqueIndices(id, item, false)
+	}
+
+	delete(bs.items, id)
+	bs.rebuildIndicesUnsafe()
+}
+
+// RemoveWithRebuild removes item and rebuilds indices
+func (bs *BaseStore[T]) RemoveWithRebuild(id int) {
+	bs.RemoveWithValidation(id)
+}
+
+// UpdateWithValidation updates item with validation and index rebuild
+func (bs *BaseStore[T]) UpdateWithValidation(id int, item *T) error {
+	return bs.AddWithValidation(id, item)
+}
+
+// UpdateWithRebuild updates item with validation and index rebuild
+func (bs *BaseStore[T]) UpdateWithRebuild(id int, item *T) error {
+	return bs.AddWithValidation(id, item)
+}
+
+// Find retrieves an item by ID
+func (bs *BaseStore[T]) Find(id int) (*T, bool) {
+	bs.mu.RLock()
+	defer bs.mu.RUnlock()
+	item, exists := bs.items[id]
+	return item, exists
+}
+
+// AllSorted returns all items using named sorted index
+func (bs *BaseStore[T]) AllSorted(indexName string) []*T {
+	bs.mu.RLock()
+	defer bs.mu.RUnlock()
+
+	if index, exists := bs.indices[indexName]; exists {
+		if sortedIDs, ok := index.([]int); ok {
+			result := make([]*T, 0, len(sortedIDs))
+			for _, id := range sortedIDs {
+				if item, exists := bs.items[id]; exists {
+					result = append(result, item)
+				}
+			}
+			return result
+		}
+	}
+
+	// Fallback: return all items by ID order
+	ids := make([]int, 0, len(bs.items))
+	for id := range bs.items {
+		ids = append(ids, id)
+	}
+	sort.Ints(ids)
+
+	result := make([]*T, 0, len(ids))
+	for _, id := range ids {
+		result = append(result, bs.items[id])
+	}
+	return result
+}
+
+// LookupByIndex finds single item using string lookup index
+func (bs *BaseStore[T]) LookupByIndex(indexName, key string) (*T, bool) {
+	bs.mu.RLock()
+	defer bs.mu.RUnlock()
+
+	if index, exists := bs.indices[indexName]; exists {
+		if lookupMap, ok := index.(map[string]int); ok {
+			if id, found := lookupMap[key]; found {
+				if item, exists := bs.items[id]; exists {
+					return item, true
+				}
+			}
+		}
+	}
+	return nil, false
+}
+
+// GroupByIndex returns items grouped by key
+func (bs *BaseStore[T]) GroupByIndex(indexName string, key any) []*T {
+	bs.mu.RLock()
+	defer bs.mu.RUnlock()
+
+	if index, exists := bs.indices[indexName]; exists {
+		switch groupMap := index.(type) {
+		case map[int][]int:
+			if intKey, ok := key.(int); ok {
+				if ids, found := groupMap[intKey]; found {
+					result := make([]*T, 0, len(ids))
+					for _, id := range ids {
+						if item, exists := bs.items[id]; exists {
+							result = append(result, item)
+						}
+					}
+					return result
+				}
+			}
+		case map[string][]int:
+			if strKey, ok := key.(string); ok {
+				if ids, found := groupMap[strKey]; found {
+					result := make([]*T, 0, len(ids))
+					for _, id := range ids {
+						if item, exists := bs.items[id]; exists {
+							result = append(result, item)
+						}
+					}
+					return result
+				}
+			}
+		}
+	}
+	return []*T{}
+}
+
+// FilterByIndex returns items matching filter criteria
+func (bs *BaseStore[T]) FilterByIndex(indexName string, filterFunc func(*T) bool) []*T {
+	bs.mu.RLock()
+	defer bs.mu.RUnlock()
+
+	var sourceIDs []int
+
+	if index, exists := bs.indices[indexName]; exists {
+		if sortedIDs, ok := index.([]int); ok {
+			sourceIDs = sortedIDs
+		}
+	}
+
+	if sourceIDs == nil {
+		for id := range bs.items {
+			sourceIDs = append(sourceIDs, id)
+		}
+		sort.Ints(sourceIDs)
+	}
+
+	var result []*T
+	for _, id := range sourceIDs {
+		if item, exists := bs.items[id]; exists && filterFunc(item) {
+			result = append(result, item)
+		}
+	}
+	return result
+}
+
+// GetNextID returns the next available ID atomically
+func (bs *BaseStore[T]) GetNextID() int {
+	bs.mu.Lock()
+	defer bs.mu.Unlock()
+	bs.maxID++
+	return bs.maxID
+}
+
+// GetByID retrieves an item by ID
+func (bs *BaseStore[T]) GetByID(id int) (*T, bool) {
+	return bs.Find(id)
+}
+
+// Add adds an item to the store (no validation)
+func (bs *BaseStore[T]) Add(id int, item *T) {
+	bs.mu.Lock()
+	defer bs.mu.Unlock()
+	bs.items[id] = item
+	if id > bs.maxID {
+		bs.maxID = id
+	}
+}
+
+// Remove removes an item from the store (no validation)
+func (bs *BaseStore[T]) Remove(id int) {
+	bs.mu.Lock()
+	defer bs.mu.Unlock()
+	delete(bs.items, id)
+}
+
+// GetAll returns all items
+func (bs *BaseStore[T]) GetAll() map[int]*T {
+	bs.mu.RLock()
+	defer bs.mu.RUnlock()
+	result := make(map[int]*T, len(bs.items))
+	maps.Copy(result, bs.items)
+	return result
+}
+
+// Clear removes all items
+func (bs *BaseStore[T]) Clear() {
+	bs.mu.Lock()
+	defer bs.mu.Unlock()
+	bs.items = make(map[int]*T)
+	bs.maxID = 0
+
+	// Clear unique indices
+	for fieldName := range bs.uniqueIndices {
+		bs.uniqueIndices[fieldName] = make(map[any]int)
+	}
+
+	bs.rebuildIndicesUnsafe()
+}
+
+// Index Builder Functions
+
+// BuildFieldLookupIndex creates index for any field by name
+func BuildFieldLookupIndex[T any](fieldName string) IndexBuilder[T] {
+	return func(allItems map[int]*T) any {
+		index := make(map[string]int)
+		for id, item := range allItems {
+			itemValue := reflect.ValueOf(item).Elem()
+			fieldValue := itemValue.FieldByName(fieldName)
+			if !fieldValue.IsValid() {
+				continue
+			}
+
+			key := fmt.Sprintf("%v", fieldValue.Interface())
+			index[key] = id
+		}
+		return index
+	}
+}
+
+// BuildStringLookupIndex creates string-to-ID mapping
+func BuildStringLookupIndex[T any](keyFunc func(*T) string) IndexBuilder[T] {
+	return func(allItems map[int]*T) any {
+		index := make(map[string]int)
+		for id, item := range allItems {
+			key := keyFunc(item)
+			index[key] = id
+		}
+		return index
+	}
+}
+
+// BuildCaseInsensitiveLookupIndex creates lowercase string-to-ID mapping
+func BuildCaseInsensitiveLookupIndex[T any](keyFunc func(*T) string) IndexBuilder[T] {
+	return func(allItems map[int]*T) any {
+		index := make(map[string]int)
+		for id, item := range allItems {
+			key := strings.ToLower(keyFunc(item))
+			index[key] = id
+		}
+		return index
+	}
+}
+
+// BuildIntGroupIndex creates int-to-[]ID mapping
+func BuildIntGroupIndex[T any](keyFunc func(*T) int) IndexBuilder[T] {
+	return func(allItems map[int]*T) any {
+		index := make(map[int][]int)
+		for id, item := range allItems {
+			key := keyFunc(item)
+			index[key] = append(index[key], id)
+		}
+
+		// Sort each group by ID
+		for key := range index {
+			sort.Ints(index[key])
+		}
+
+		return index
+	}
+}
+
+// BuildStringGroupIndex creates string-to-[]ID mapping
+func BuildStringGroupIndex[T any](keyFunc func(*T) string) IndexBuilder[T] {
+	return func(allItems map[int]*T) any {
+		index := make(map[string][]int)
+		for id, item := range allItems {
+			key := keyFunc(item)
+			index[key] = append(index[key], id)
+		}
+
+		// Sort each group by ID
+		for key := range index {
+			sort.Ints(index[key])
+		}
+
+		return index
+	}
+}
+
+// BuildSortedListIndex creates sorted []ID list
+func BuildSortedListIndex[T any](sortFunc func(*T, *T) bool) IndexBuilder[T] {
+	return func(allItems map[int]*T) any {
+		ids := make([]int, 0, len(allItems))
+		for id := range allItems {
+			ids = append(ids, id)
+		}
+
+		sort.Slice(ids, func(i, j int) bool {
+			return sortFunc(allItems[ids[i]], allItems[ids[j]])
+		})
+
+		return ids
+	}
+}
+
+// BuildFilteredIntGroupIndex creates int-to-[]ID mapping for items passing filter
+func BuildFilteredIntGroupIndex[T any](filterFunc func(*T) bool, keyFunc func(*T) int) IndexBuilder[T] {
+	return func(allItems map[int]*T) any {
+		index := make(map[int][]int)
+		for id, item := range allItems {
+			if filterFunc(item) {
+				key := keyFunc(item)
+				index[key] = append(index[key], id)
+			}
+		}
+
+		// Sort each group by ID
+		for key := range index {
+			sort.Ints(index[key])
+		}
+
+		return index
+	}
+}
+
+// BuildFilteredStringGroupIndex creates string-to-[]ID mapping for items passing filter
+func BuildFilteredStringGroupIndex[T any](filterFunc func(*T) bool, keyFunc func(*T) string) IndexBuilder[T] {
+	return func(allItems map[int]*T) any {
+		index := make(map[string][]int)
+		for id, item := range allItems {
+			if filterFunc(item) {
+				key := keyFunc(item)
+				index[key] = append(index[key], id)
+			}
+		}
+
+		// Sort each group by ID
+		for key := range index {
+			sort.Ints(index[key])
+		}
+
+		return index
+	}
+}
+
+// NewSingleton creates singleton store pattern with sync.Once
+func NewSingleton[S any](initFunc func() *S) func() *S {
+	var store *S
+	var once sync.Once
+
+	return func() *S {
+		once.Do(func() {
+			store = initFunc()
+		})
+		return store
+	}
+}