Unit of Work Pattern

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// Entity state enumeration
enum EntityState {
    Transient,   // Not tracked
    Added,       // New, pending INSERT
    Clean,       // Loaded, unchanged
    Dirty,       // Modified, pending UPDATE
    Deleted      // Pending DELETE
}
 
// Entity entry - wrapper that tracks state
interface EntityEntry<T = unknown> {
    entity: T;
    state: EntityState;
    originalValues: Map<string, unknown> | null;  // For dirty checking
    entityType: EntityType;
}
 
// Entity type metadata
interface EntityType {
    name: string;
    tableName: string;
    keyProperty: string;
    properties: PropertyMetadata[];
    relationships: RelationshipMetadata[];
}
 
// State tracker interface
interface StateTracker {
    getEntry<T>(entity: T): EntityEntry<T> | undefined;
    getState<T>(entity: T): EntityState;
    setState<T>(entity: T, state: EntityState): void;
    getEntriesByState(state: EntityState): EntityEntry[];
    hasChanges(): boolean;
}

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// ❌ WITHOUT Identity Map - Disaster waiting to happen
async function processOrder(orderId: string) {
    // First query: get order for display
    const orderForDisplay = await orderRepository.findById(orderId);
    console.log(`Order total: ${orderForDisplay.total}`);
    
    // Second query: get same order for modification
    const orderForUpdate = await orderRepository.findById(orderId);
    orderForUpdate.addItem(newItem);
    orderForUpdate.recalculateTotal();  // Now total = 150
    await orderRepository.save(orderForUpdate);
    
    // Problem: orderForDisplay.total is still 100 (stale)
    // Even worse: if we save orderForDisplay later, we overwrite!
    
    // orderForDisplay and orderForUpdate are DIFFERENT objects
    console.log(orderForDisplay === orderForUpdate); // false!
}
 
// ✅ WITH Identity Map - Same object always
async function processOrderSafe(orderId: string, unitOfWork: IUnitOfWork) {
    const orderRepo = unitOfWork.orderRepository;
    
    // First query
    const orderForDisplay = await orderRepo.findById(orderId);
    
    // Second query - returns SAME INSTANCE from identity map
    const orderForUpdate = await orderRepo.findById(orderId);
    
    console.log(orderForDisplay === orderForUpdate); // true!
    
    orderForUpdate.addItem(newItem);  // Modifies the single instance
    // orderForDisplay sees the change too - same object!
}

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class IdentityMap {
    // Map: EntityTypeName -> Map<EntityId, EntityEntry>
    private entities: Map<string, Map<string, EntityEntry>> = new Map();
    
    /**
     * Create a unique key for an entity
     */
    private getKey(entityType: string, id: unknown): string {
        // Handle composite keys
        if (Array.isArray(id)) {
            return id.map(String).join('::');
        }
        return String(id);
    }
    
    /**
     * Check if entity exists in the map
     */
    contains(entityType: string, id: unknown): boolean {
        const typeMap = this.entities.get(entityType);
        if (!typeMap) return false;
        return typeMap.has(this.getKey(entityType, id));
    }
    
    /**
     * Get entity from the map
     */
    get<T>(entityType: string, id: unknown): EntityEntry<T> | undefined {
        const typeMap = this.entities.get(entityType);
        if (!typeMap) return undefined;
        return typeMap.get(this.getKey(entityType, id)) as EntityEntry<T> | undefined;
    }
    
    /**
     * Add entity to the map
     */
    add<T>(entityType: string, id: unknown, entry: EntityEntry<T>): void {
        let typeMap = this.entities.get(entityType);
        if (!typeMap) {
            typeMap = new Map();
            this.entities.set(entityType, typeMap);
        }
        
        const key = this.getKey(entityType, id);
        if (typeMap.has(key)) {
            throw new Error(
                `Entity of type ${entityType} with id ${key} already exists in identity map`
            );
        }
        
        typeMap.set(key, entry as EntityEntry);
    }
    
    /**
     * Remove entity from the map
     */
    remove(entityType: string, id: unknown): boolean {
        const typeMap = this.entities.get(entityType);
        if (!typeMap) return false;
        return typeMap.delete(this.getKey(entityType, id));
    }
    
    /**
     * Get all entries of a given state
     */
    getByState(state: EntityState): EntityEntry[] {
        const results: EntityEntry[] = [];
        for (const typeMap of this.entities.values()) {
            for (const entry of typeMap.values()) {
                if (entry.state === state) {
                    results.push(entry);
                }
            }
        }
        return results;
    }
    
    /**
     * Get all tracked entities
     */
    getAll(): EntityEntry[] {
        const results: EntityEntry[] = [];
        for (const typeMap of this.entities.values()) {
            for (const entry of typeMap.values()) {
                results.push(entry);
            }
        }
        return results;
    }
    
    /**
     * Clear all tracked entities
     */
    clear(): void {
        this.entities.clear();
    }
}

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class ChangeTracker {
    private identityMap: IdentityMap = new IdentityMap();
    private metadata: EntityMetadataRegistry;
    
    constructor(metadata: EntityMetadataRegistry) {
        this.metadata = metadata;
    }
    
    /**
     * Track a new entity (will be INSERTed on commit)
     */
    trackNew<T extends object>(entity: T): void {
        const entityType = this.metadata.getTypeFor(entity);
        const id = this.extractId(entity, entityType);
        
        // For new entities, id might be null/undefined (auto-generated)
        // Use a temporary id until commit
        const trackingId = id ?? this.generateTempId();
        
        const entry: EntityEntry<T> = {
            entity,
            state: EntityState.Added,
            originalValues: null,
            entityType
        };
        
        this.identityMap.add(entityType.name, trackingId, entry);
    }
    
    /**
     * Track an entity loaded from database (starts as Clean)
     */
    trackLoaded<T extends object>(entity: T): void {
        const entityType = this.metadata.getTypeFor(entity);
        const id = this.extractId(entity, entityType);
        
        // Check if already tracked
        if (this.identityMap.contains(entityType.name, id)) {
            return; // Already tracking this entity
        }
        
        const entry: EntityEntry<T> = {
            entity,
            state: EntityState.Clean,
            originalValues: this.takeSnapshot(entity, entityType),
            entityType
        };
        
        this.identityMap.add(entityType.name, id, entry);
    }
    
    /**
     * Mark entity for deletion
     */
    trackDeleted<T extends object>(entity: T): void {
        const entityType = this.metadata.getTypeFor(entity);
        const id = this.extractId(entity, entityType);
        
        const entry = this.identityMap.get<T>(entityType.name, id);
        
        if (!entry) {
            throw new Error('Cannot delete entity that is not tracked');
        }
        
        if (entry.state === EntityState.Added) {
            // New entity being deleted - just remove from tracking
            this.identityMap.remove(entityType.name, id);
        } else {
            entry.state = EntityState.Deleted;
        }
    }
    
    /**
     * Take a snapshot of entity property values
     */
    private takeSnapshot(entity: object, entityType: EntityType): Map<string, unknown> {
        const snapshot = new Map<string, unknown>();
        
        for (const prop of entityType.properties) {
            const value = (entity as any)[prop.name];
            // Deep clone for objects/arrays
            snapshot.set(prop.name, this.cloneValue(value));
        }
        
        return snapshot;
    }
    
    /**
     * Detect changes by comparing current values to snapshot
     */
    detectChanges(): void {
        for (const entry of this.identityMap.getByState(EntityState.Clean)) {
            if (this.hasChanges(entry)) {
                entry.state = EntityState.Dirty;
            }
        }
    }
    
    /**
     * Check if entity has changed since loaded
     */
    private hasChanges(entry: EntityEntry): boolean {
        if (!entry.originalValues) return false;
        
        for (const prop of entry.entityType.properties) {
            const currentValue = (entry.entity as any)[prop.name];
            const originalValue = entry.originalValues.get(prop.name);
            
            if (!this.valuesEqual(currentValue, originalValue)) {
                return true;
            }
        }
        
        return false;
    }
    
    /**
     * Get all pending changes grouped by operation type
     */
    getPendingChanges(): PendingChanges {
        this.detectChanges();
        
        return {
            inserts: this.identityMap.getByState(EntityState.Added),
            updates: this.identityMap.getByState(EntityState.Dirty),
            deletes: this.identityMap.getByState(EntityState.Deleted)
        };
    }
    
    /**
     * Mark all entities as clean after successful commit
     */
    acceptChanges(): void {
        for (const entry of this.identityMap.getAll()) {
            if (entry.state === EntityState.Deleted) {
                // Remove deleted entities
                const id = this.extractId(entry.entity as object, entry.entityType);
                this.identityMap.remove(entry.entityType.name, id);
            } else if (entry.state === EntityState.Added || entry.state === EntityState.Dirty) {
                // Reset to clean
                entry.state = EntityState.Clean;
                entry.originalValues = this.takeSnapshot(
                    entry.entity as object, 
                    entry.entityType
                );
            }
        }
    }
    
    /**
     * Discard all changes and restore original values
     */
    rejectChanges(): void {
        for (const entry of this.identityMap.getAll()) {
            if (entry.state === EntityState.Added) {
                // Remove new entities
                const id = this.extractId(entry.entity as object, entry.entityType);
                this.identityMap.remove(entry.entityType.name, id);
            } else if (entry.state === EntityState.Dirty) {
                // Restore original values
                this.restoreSnapshot(entry);
                entry.state = EntityState.Clean;
            } else if (entry.state === EntityState.Deleted) {
                // Undelete
                entry.state = EntityState.Clean;
            }
        }
    }
    
    private restoreSnapshot(entry: EntityEntry): void {
        if (!entry.originalValues) return;
        
        for (const [prop, value] of entry.originalValues) {
            (entry.entity as any)[prop] = this.cloneValue(value);
        }
    }
    
    // Helper methods
    private extractId(entity: object, entityType: EntityType): unknown {
        return (entity as any)[entityType.keyProperty];
    }
    
    private generateTempId(): string {
        return `temp_${Date.now()}_${Math.random().toString(36).substr(2, 9)}`;
    }
    
    private cloneValue(value: unknown): unknown {
        if (value === null || value === undefined) return value;
        if (typeof value !== 'object') return value;
        if (value instanceof Date) return new Date(value.getTime());
        return JSON.parse(JSON.stringify(value));
    }
    
    private valuesEqual(a: unknown, b: unknown): boolean {
        if (a === b) return true;
        if (a instanceof Date && b instanceof Date) {
            return a.getTime() === b.getTime();
        }
        if (typeof a === 'object' && typeof b === 'object') {
            return JSON.stringify(a) === JSON.stringify(b);
        }
        return false;
    }
}
 
interface PendingChanges {
    inserts: EntityEntry[];
    updates: EntityEntry[];
    deletes: EntityEntry[];
}

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// Domain model
class Order {
    id: string;
    customerId: string;  // FK to Customer
    items: OrderItem[];
}
 
class OrderItem {
    id: string;
    orderId: string;  // FK to Order
    productId: string;  // FK to Product
}
 
// If we try to insert Order before Customer exists: FK violation!
// If we try to insert OrderItem before Order exists: FK violation!
 
// Correct INSERT order:
// 1. Customer (if new)
// 2. Order (depends on Customer)
// 3. Product (if new, no dependencies)
// 4. OrderItem (depends on Order AND Product)
 
// Correct DELETE order (opposite!):
// 1. OrderItem (children first)
// 2. Order
// 3. Customer (only if no other orders)

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class DependencyResolver {
    private metadata: EntityMetadataRegistry;
    
    constructor(metadata: EntityMetadataRegistry) {
        this.metadata = metadata;
    }
    
    /**
     * Order entity entries for INSERT operations
     * Parents must be inserted before children
     */
    orderForInsert(entries: EntityEntry[]): EntityEntry[] {
        const graph = this.buildDependencyGraph(entries);
        return this.topologicalSort(graph, entries);
    }
    
    /**
     * Order entity entries for DELETE operations
     * Children must be deleted before parents (reverse of insert)
     */
    orderForDelete(entries: EntityEntry[]): EntityEntry[] {
        const insertOrder = this.orderForInsert(entries);
        return insertOrder.reverse();
    }
    
    /**
     * Build a graph of type dependencies
     */
    private buildDependencyGraph(entries: EntityEntry[]): Map<string, Set<string>> {
        const graph = new Map<string, Set<string>>();
        
        // Initialize nodes
        for (const entry of entries) {
            const typeName = entry.entityType.name;
            if (!graph.has(typeName)) {
                graph.set(typeName, new Set());
            }
        }
        
        // Add edges based on foreign key relationships
        for (const entry of entries) {
            const typeName = entry.entityType.name;
            
            for (const rel of entry.entityType.relationships) {
                if (rel.type === 'many-to-one' || rel.type === 'one-to-one') {
                    // This type depends on the related type
                    const dependsOn = rel.targetType;
                    if (graph.has(dependsOn)) {
                        graph.get(typeName)!.add(dependsOn);
                    }
                }
            }
        }
        
        return graph;
    }
    
    /**
     * Kahn's algorithm for topological sort
     */
    private topologicalSort(
        graph: Map<string, Set<string>>, 
        entries: EntityEntry[]
    ): EntityEntry[] {
        // Calculate in-degrees
        const inDegree = new Map<string, number>();
        for (const [node, deps] of graph) {
            if (!inDegree.has(node)) {
                inDegree.set(node, 0);
            }
            for (const dep of deps) {
                inDegree.set(dep, (inDegree.get(dep) || 0));
            }
        }
        
        for (const [node, deps] of graph) {
            for (const dep of deps) {
                // node depends on dep, so node's in-degree increases
                // Actually, we want nodes with NO dependencies first
            }
        }
        
        // Nodes with no dependencies come first
        const queue: string[] = [];
        for (const [node] of graph) {
            const deps = graph.get(node)!;
            if (deps.size === 0) {
                queue.push(node);
            }
        }
        
        const ordered: string[] = [];
        const processed = new Set<string>();
        
        while (queue.length > 0) {
            const current = queue.shift()!;
            if (processed.has(current)) continue;
            
            processed.add(current);
            ordered.push(current);
            
            // Find nodes that depend on current
            for (const [node, deps] of graph) {
                if (deps.has(current) && !processed.has(node)) {
                    // Check if all dependencies are now processed
                    const allDepsProcessed = [...deps].every(d => processed.has(d));
                    if (allDepsProcessed) {
                        queue.push(node);
                    }
                }
            }
        }
        
        // Detect cycles
        if (processed.size !== graph.size) {
            throw new Error('Circular dependency detected in entity graph');
        }
        
        // Map type order to entry order
        const typeOrder = new Map(ordered.map((type, index) => [type, index]));
        
        return entries.sort((a, b) => {
            const orderA = typeOrder.get(a.entityType.name) ?? 0;
            const orderB = typeOrder.get(b.entityType.name) ?? 0;
            return orderA - orderB;
        });
    }
}

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interface IUnitOfWork {
    // Repository access
    readonly orders: IOrderRepository;
    readonly products: IProductRepository;
    readonly customers: ICustomerRepository;
    
    // Transaction control
    begin(): Promise<void>;
    commit(): Promise<void>;
    rollback(): Promise<void>;
    
    // Change tracking
    registerNew<T extends object>(entity: T): void;
    registerDirty<T extends object>(entity: T): void;
    registerDeleted<T extends object>(entity: T): void;
    
    // Status
    hasChanges(): boolean;
}
 
class UnitOfWork implements IUnitOfWork {
    private connection: DatabaseConnection;
    private transaction: DatabaseTransaction | null = null;
    private changeTracker: ChangeTracker;
    private dependencyResolver: DependencyResolver;
    private sqlGenerator: SqlGenerator;
    private isDisposed: boolean = false;
    
    // Lazy-initialized repositories
    private _orders: OrderRepository | null = null;
    private _products: ProductRepository | null = null;
    private _customers: CustomerRepository | null = null;
    
    constructor(
        connectionFactory: () => Promise<DatabaseConnection>,
        metadata: EntityMetadataRegistry
    ) {
        this.connection = connectionFactory();
        this.changeTracker = new ChangeTracker(metadata);
        this.dependencyResolver = new DependencyResolver(metadata);
        this.sqlGenerator = new SqlGenerator(metadata);
    }
    
    // Repository accessors - all share the same change tracker
    get orders(): IOrderRepository {
        if (!this._orders) {
            this._orders = new OrderRepository(this.connection, this.changeTracker);
        }
        return this._orders;
    }
    
    get products(): IProductRepository {
        if (!this._products) {
            this._products = new ProductRepository(this.connection, this.changeTracker);
        }
        return this._products;
    }
    
    get customers(): ICustomerRepository {
        if (!this._customers) {
            this._customers = new CustomerRepository(this.connection, this.changeTracker);
        }
        return this._customers;
    }
    
    // Change registration (delegates to change tracker)
    registerNew<T extends object>(entity: T): void {
        this.ensureNotDisposed();
        this.changeTracker.trackNew(entity);
    }
    
    registerDirty<T extends object>(entity: T): void {
        this.ensureNotDisposed();
        // Note: With snapshot-based tracking, this might not be needed
        // The change tracker can auto-detect dirty entities
    }
    
    registerDeleted<T extends object>(entity: T): void {
        this.ensureNotDisposed();
        this.changeTracker.trackDeleted(entity);
    }
    
    hasChanges(): boolean {
        const changes = this.changeTracker.getPendingChanges();
        return changes.inserts.length > 0 
            || changes.updates.length > 0 
            || changes.deletes.length > 0;
    }
    
    // Transaction management
    async begin(): Promise<void> {
        this.ensureNotDisposed();
        if (this.transaction) {
            throw new Error('Transaction already in progress');
        }
        this.transaction = await this.connection.beginTransaction();
    }
    
    async commit(): Promise<void> {
        this.ensureNotDisposed();
        
        try {
            // Get all pending changes
            const changes = this.changeTracker.getPendingChanges();
            
            // Order operations for dependency constraints
            const orderedInserts = this.dependencyResolver.orderForInsert(changes.inserts);
            const orderedDeletes = this.dependencyResolver.orderForDelete(changes.deletes);
            
            // Execute in correct order: deletes first (optional), then inserts, then updates
            // Some strategies do inserts first, then updates, then deletes
            // Order depends on your constraint handling preference
            
            // Execute INSERTs
            for (const entry of orderedInserts) {
                const sql = this.sqlGenerator.generateInsert(entry);
                const result = await this.executeWithTransaction(sql);
                
                // If ID was auto-generated, update the entity
                if (result.insertedId && !this.hasId(entry.entity)) {
                    this.setId(entry.entity, entry.entityType, result.insertedId);
                }
            }
            
            // Execute UPDATEs
            for (const entry of changes.updates) {
                const sql = this.sqlGenerator.generateUpdate(entry);
                await this.executeWithTransaction(sql);
            }
            
            // Execute DELETEs
            for (const entry of orderedDeletes) {
                const sql = this.sqlGenerator.generateDelete(entry);
                await this.executeWithTransaction(sql);
            }
            
            // Commit transaction if we started one
            if (this.transaction) {
                await this.transaction.commit();
                this.transaction = null;
            }
            
            // Mark all entities as clean
            this.changeTracker.acceptChanges();
            
        } catch (error) {
            // Rollback on any failure
            await this.rollback();
            throw error;
        }
    }
    
    async rollback(): Promise<void> {
        if (this.transaction) {
            await this.transaction.rollback();
            this.transaction = null;
        }
        
        // Restore in-memory state
        this.changeTracker.rejectChanges();
    }
    
    private async executeWithTransaction(sql: SqlStatement): Promise<QueryResult> {
        if (this.transaction) {
            return await this.transaction.execute(sql.text, sql.parameters);
        }
        return await this.connection.execute(sql.text, sql.parameters);
    }
    
    private ensureNotDisposed(): void {
        if (this.isDisposed) {
            throw new Error('Unit of Work has been disposed');
        }
    }
    
    private hasId(entity: object): boolean {
        // Check if entity has an ID set
        return (entity as any).id != null;
    }
    
    private setId(entity: object, entityType: EntityType, id: unknown): void {
        (entity as any)[entityType.keyProperty] = id;
    }
    
    // Cleanup
    async dispose(): Promise<void> {
        if (this.isDisposed) return;
        
        if (this.transaction) {
            await this.rollback();
        }
        
        await this.connection.close();
        this.isDisposed = true;
    }
}

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class OrderRepository implements IOrderRepository {
    private connection: DatabaseConnection;
    private changeTracker: ChangeTracker;
    
    constructor(connection: DatabaseConnection, changeTracker: ChangeTracker) {
        this.connection = connection;
        this.changeTracker = changeTracker;
    }
    
    async findById(id: string): Promise<Order | null> {
        // CRITICAL: Check identity map first!
        const tracked = this.changeTracker.getTracked<Order>('Order', id);
        if (tracked) {
            return tracked;
        }
        
        // Not in identity map, query database
        const result = await this.connection.query(
            'SELECT * FROM orders WHERE id = $1',
            [id]
        );
        
        if (result.rows.length === 0) {
            return null;
        }
        
        // Map row to entity
        const order = this.mapRowToOrder(result.rows[0]);
        
        // Register as clean (loaded from DB)
        this.changeTracker.trackLoaded(order);
        
        return order;
    }
    
    async findByCustomer(customerId: string): Promise<Order[]> {
        const result = await this.connection.query(
            'SELECT * FROM orders WHERE customer_id = $1',
            [customerId]
        );
        
        return result.rows.map(row => {
            // Check identity map for each row
            const existing = this.changeTracker.getTracked<Order>('Order', row.id);
            if (existing) {
                return existing;
            }
            
            const order = this.mapRowToOrder(row);
            this.changeTracker.trackLoaded(order);
            return order;
        });
    }
    
    add(order: Order): void {
        // Register as new with change tracker
        this.changeTracker.trackNew(order);
    }
    
    remove(order: Order): void {
        this.changeTracker.trackDeleted(order);
    }
    
    private mapRowToOrder(row: any): Order {
        return new Order({
            id: row.id,
            customerId: row.customer_id,
            total: parseFloat(row.total),
            status: row.status,
            createdAt: row.created_at
        });
    }
}