关于TypeORM中OneToMany与ManyToOne默认抓取类型、延迟加载与Promise的关系及与JPA默认抓取类型是否一致的技术疑问

Great question! Let's unpack each part of your inquiry step by step—there's some nuance here with TypeORM's loading strategies, especially when comparing to JPA.

TypeORM's Default Fetch Types for OneToMany & ManyToOne #

• @OneToMany: By default, this association uses lazy loading. But as the official docs specify, to actually enable this lazy behavior, you must declare the associated property as a Promise (e.g., posts: Promise<Post[]>). If you skip the Promise (using something like posts: Post[]), TypeORM is supposed to fall back to eager loading. That said, if your testing showed it was still lazy, a few things might be going on:
  • You could be using an older TypeORM version where this behavior was unenforced.
  • Maybe you explicitly set lazy: true on the @OneToMany decorator without using a Promise—the docs warn against this, since lazy loading relies on proxy objects that need the Promise type hint to work correctly.
• @ManyToOne: This association defaults to eager loading. No Promise is required here: when you fetch the owning entity, the associated related entity will automatically be loaded in the same query via a join.

Lazy Loading Without Promises: Why Your Tests Might Show Different Behavior #

The official docs are clear: lazy loading requires Promise-typed properties because TypeORM uses proxy objects to defer the database query until you access the property. If you don't use a Promise, TypeORM can't create that proxy, so it should switch to eager loading. If your results don't match this, double-check:

• Did you accidentally override the default with lazy: false or eager: false on the association decorator?
• Are you working in a transaction or query runner context that's altering loading behavior?
• Could you be confusing "lazy loading" with not explicitly joining the association in your query? Eager loading should happen automatically without needing to specify the relations option.

TypeORM vs. JPA Default Fetch Strategies #

For the most part, TypeORM aligns closely with JPA's defaults, with one small exception for @OneToOne associations:

So the core defaults for OneToMany and ManyToOne match JPA exactly—TypeORM just adds the requirement of using Promise to activate the lazy loading behavior for OneToMany.

内容的提问来源于stack exchange，提问作者momo