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The Criteria pattern in NestJS: what a client may ask for is a file, not a signature

The Criteria pattern in NestJS One single way to filter, sort and paginate any list. Five parameters and a find() The example running through this article is a library catalogue. A book stores this: // src/book/book.schema.ts @ Schema ({ timestamps : true }) export class Book { @ Prop () title : string ; @ Prop ({ type : Types . ObjectId , ref : " Author " }) author : Types . ObjectId ; @ Prop () publishedAt : Date ; @ Prop () copies : number ; // copies on the shelf @ Prop () available : boolean ; @ Prop () acquisitionPrice : number ; // what it cost us: internal, never published } The author's name is not here: it lives in the authors collection, on the other side of that reference. And the screen consuming the catalogue is a table with a search box, per-column filters and pagination. The endpoint feeding it is written once and grows by accretion. It starts returning a page with a fixed order, and by the time the table has all its filters it has become this: // src/book/book.controller.ts @ Controller ( " books " ) export class BookController { constructor ( @ InjectModel ( Book . name ) private readonly model : Model < BookDocument > , ) {} @ Get () async getAll ( @ Query ( " title " ) title ?: string , @ Query ( " available " ) available ?: string , @ Query ( " minCopies " ) minCopies ?: string , @ Query ( " sortBy " ) sortBy ?: string , @ Query ( " page " ) page ?: string , ) { const filter : FilterQuery < BookDocument > = {}; if ( title ) { filter . title = { $regex : title , $options : " i " }; } if ( available ) { filter . available = available === " true " ; } if ( minCopies ) { filter . copies = { $gte : Number ( minCopies ) }; } const current = Number ( page ?? 1 ); const [ items , total ] = await Promise . all ([ this . model . find ( filter ) . sort ({ [ sortBy ?? " createdAt " ]: - 1 }) . skip (( current - 1 ) * 20 ) . limit ( 20 ), this . model . countDocuments ( filter ), ]); return { items : items , total : total , page : current }; } } There are co

2026-08-21 原文 →
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El patrón Criteria en NestJS: lo que un cliente puede pedir es un archivo, no una firma

Cinco parámetros y un find() El ejemplo de todo el artículo es el catálogo de una biblioteca. Un libro guarda esto: // src/book/book.schema.ts @ Schema ({ timestamps : true }) export class Book { @ Prop () title : string ; @ Prop ({ type : Types . ObjectId , ref : " Author " }) author : Types . ObjectId ; @ Prop () publishedAt : Date ; @ Prop () copies : number ; // ejemplares en la estantería @ Prop () available : boolean ; @ Prop () acquisitionPrice : number ; // lo que costó adquirirlo: interno, no se publica } El nombre del autor no está aquí: vive en la colección authors , al otro lado de esa referencia. Y la pantalla que consume el catálogo es una tabla con buscador, filtros por columna y paginación. El endpoint que la alimenta se escribe una vez y crece por acumulación. Empieza devolviendo una página con un orden fijo, y para cuando la tabla tiene todos sus filtros ha llegado a esto: // src/book/book.controller.ts @ Controller ( " books " ) export class BookController { constructor ( @ InjectModel ( Book . name ) private readonly model : Model < BookDocument > , ) {} @ Get () async getAll ( @ Query ( " title " ) title ?: string , @ Query ( " available " ) available ?: string , @ Query ( " minCopies " ) minCopies ?: string , @ Query ( " sortBy " ) sortBy ?: string , @ Query ( " page " ) page ?: string , ) { const filter : FilterQuery < BookDocument > = {}; if ( title ) { filter . title = { $regex : title , $options : " i " }; } if ( available ) { filter . available = available === " true " ; } if ( minCopies ) { filter . copies = { $gte : Number ( minCopies ) }; } const current = Number ( page ?? 1 ); const [ items , total ] = await Promise . all ([ this . model . find ( filter ) . sort ({ [ sortBy ?? " createdAt " ]: - 1 }) . skip (( current - 1 ) * 20 ) . limit ( 20 ), this . model . countDocuments ( filter ), ]); return { items : items , total : total , page : current }; } } El método tiene decisiones correctas dentro: el total sale del mismo filtro que los

2026-08-21 原文 →
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LISKOV SUBSTITUTION PRINCIPLE

A parent class must be able to be substituted by its child classes without breaking the application. In practice, this helps to organize the idea of inheritance, as it prevents us from extending a parent class only to later remove an already implemented method or do a “throw new Error(‘Not implemented’)”. Making us much more careful during planning. THE BIGGEST SYMPTOM OF ERROR Unfortunately, it is a symptom that appears late, but it is exactly when we are going to make a new implementation. You realize you violated Liskov when you are going to build a class or subclass and need to purposely throw an error in the implementation of a method. Exactly because that method shouldn't be there, but it is. A BAD EXAMPLE For example, in a delivery system. In this case, the “Delivery” class should be the parent/base for the other implementations. But the ‘MotoboyDelivery’ class breaks this. Code Example: // BAD: The subclass breaks the parent class contract. class Delivery { public calculateShipping (): number { return 15.0 ; } public getTrackingCode (): string { return " TRK123456789 " ; } } class MotoboyDelivery extends Delivery { public calculateShipping (): number { return 8.0 ; } // ERROR! There is no tracking code. public getTrackingCode (): string { throw new Error ( " Motoboys do not have a tracking code. " ); } } THE SOLUTION For those who do not yet know the 'Liskov Substitution Principle', it might seem that fitting in a sequence of 'if's is the solution. But in reality, the ideal path is to rethink how this abstraction is built. A good guiding principle is to think that a child class must always be able to take the place of the parent, without breaking the application. A GOOD EXAMPLE Still in the delivery system. ‘Delivery’ now has ‘TrackableDelivery’ in the middle of the way. With this, each “leaf”/edge of the application inherits what makes the most sense and nothing is broken. Code Example: interface Delivery { calculateShipping (): number ; } interface Trackab

2026-08-20 原文 →
AI 资讯

Design Patterns: Reusable Solutions to Recurring Problems

Design Patterns: Reusable Solutions to Recurring Problems A practical guide to classic design patterns in C#/.NET — Factory, Singleton, Repository, Strategy, and Mediator — covering what problem each one actually solves, working implementations, common .NET-specific variations, and honest guidance on when each pattern earns its complexity versus when it's unnecessary ceremony. Table of Contents Introduction Factory Pattern Singleton Pattern Repository Pattern Strategy Pattern Mediator Pattern How These Patterns Combine in Practice Patterns vs. Over-Engineering Common Pitfalls Quick Reference Table Conclusion Introduction Design patterns are named, reusable solutions to problems that recur often enough across software projects that giving them a shared name and shape is genuinely useful — not because the specific code is copy-pasteable, but because the name lets developers communicate a design intent quickly ("just make it a Strategy") instead of re-explaining the same structural idea from scratch every time. This guide covers five of the most commonly used patterns in .NET codebases, with working C# examples, and — consistent with this series' recurring theme — honest guidance on when each pattern is solving a genuine problem versus adding structure a simpler solution wouldn't need. // A pattern name compresses a whole design conversation into one word "Just inject an IPaymentStrategy and pick the implementation based on the payment method" // ← Strategy "Wrap the whole multi-step checkout process behind a single mediator call" // ← Mediator 1. Factory Pattern The problem: object creation logic that doesn't belong at the call site // ❌ The caller needs to know about every concrete shipping provider and how to construct each one IShippingProvider provider = order . Region switch { "US" => new UpsShippingProvider ( apiKey , region ), "EU" => new DhlShippingProvider ( apiKey , endpoint ), "APAC" => new FedExShippingProvider ( apiKey , credentials ), _ => throw new NotS

2026-08-18 原文 →
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The Matte Learns Only Inside the Band

A bad cutout rarely announces itself as a bad cutout. The car lands on a new backdrop, the paint looks clean, then a thin piece is gone. An antenna. A tire lip. The dark seam under a rocker panel. The complaint that comes back is never technical. The vehicle looks wrong. I wanted the last correction stage to fix fuzzy edges without handing it the whole car to rewrite. That sounds like a small distinction. It stops being small the first time a model improves one boundary and quietly damages another. So the rule is physical. Edit the uncertain strip. Leave the settled area alone. This is Part 2. Part 1, "Negative Space Is a Label", was about supervision: what the pixels beside an object teach a model, and why a shadow touching a tire has to be labeled as evidence against foreground. This one moves from training to runtime. A mask already exists. Where is a learned stage allowed to act? 1. The contract lives in the band CarSegNet is the research implementation here. Its pipeline module splits the route by media type, and the docstring says the design more clearly than any diagram I could draw after the fact. Stills run SAM 3 text concept, then NSJ alpha, then composite. A detector box prompt and a depth prior are optional inputs. Video runs SAM 3.1 multiplex propagation, per-frame NSJ with temporal handling, a depth-parallax plate, composite, encode. The list matters less than the handoff. SAM gives a semantic prior. NSJ receives a trimap band. The compositor receives a matte only after the prior and the refiner have each done bounded work. flowchart TD image[Vehicle Image] segment[Concept Mask] trimap[Trimap Band] refiner[NSJ Alpha Refiner] depth[Depth Prior] composite[Showroom Composite] frozen[Prior Frozen Outside Band] image --> segment segment --> trimap trimap --> refiner image --> depth depth --> refiner refiner --> composite segment -.-> frozen frozen --> composite The diagram is a contract. It is not a model zoo. The refiner edits the uncertain strip. The sema

2026-08-18 原文 →
AI 资讯

Presentation: From Thousands to One: Building LLM-Powered Selection Systems

Jendrik Jördening shares practical engineering strategies for integrating LLMs into production pipelines. He discusses overcoming non-determinism, restricting schemas, separating semantic text extraction from deterministic code, and validating choices using discriminator models. Learn how to structure LLMs with an MVC approach to ensure database integrity, observability, and system reliability. By Jendrik Jördening

2026-08-17 原文 →