Entity save fails after lazy loading relations

[AnderoKoplus]

Issue type:

[ ] question
[x] bug report
[ ] feature request
[ ] documentation issue

Database system/driver:

[ ] cordova
[ ] mongodb
[ ] mssql
[ ] mysql / mariadb
[ ] oracle
[x] postgres
[ ] sqlite
[ ] sqljs
[ ] react-native
[ ] expo

TypeORM version:

[x] latest
[ ] @next
[x] 0.2.12 and 0.2.13 (or put your version here)

Steps to reproduce or a small repository showing the problem:

import { Entity, JoinTable, ManyToMany, PrimaryGeneratedColumn } from "typeorm";

@Entity()
export class FooBar {
    @PrimaryGeneratedColumn()
    public id!: number;

    @ManyToMany((type) => FooBar, (fooBar) => fooBar.competitors)
    @JoinTable()
    public competitors!: Promise<FooBar[]>;
}

    const fooBar = await getManager().getRepository(FooBar).findOne(1);
    const c = await fooBar.competitors; // lazy loading causes ORM to fail
    const a = await getManager().getRepository(FooBar).save(fooBar);

TypeError: Cannot read property 'find' of undefined
    at persistence\subject-builder\ManyToManySubjectBuilder.ts:152:75
    at Array.forEach (<anonymous>)
    at ManyToManySubjectBuilder.buildForSubjectRelation (persistence\subject-builder\ManyToManySubjectBuilder.ts:117:25)
    at persistence\subject-builder\ManyToManySubjectBuilder.ts:45:22
    at Array.forEach (<anonymous>)
    at persistence\subject-builder\ManyToManySubjectBuilder.ts:39:50
    at Array.forEach (<anonymous>)
    at ManyToManySubjectBuilder.build (persistence\subject-builder\ManyToManySubjectBuilder.ts:32:23)
    at EntityPersistExecutor.<anonymous> (persistence\EntityPersistExecutor.ts:106:64)
    at step (node_modules\tslib\tslib.js:133:27)

it works with 0.2.11

[ghost]

Lazy-Loading with Promises was abandonded in the end of 2018. There sadly will be no further support and i also do have mny problems with Lazy-Loading, which seem to be never solved.
See this comment as ref:
#2902 (comment)

[johannesschobel]

Wait.. do you say, that lazy-loading of relationships is deprecated? 😱

[ghost]

It's not that simple, i now know...
See this: #3703
I personally will abandon lazyloading and just switch to eager loading. You can still control if the relations are loaded or not by putting "relations: ['myRelation']" into the query parameters...

[johannesschobel]

hm, good to know.. i come from the "Laravel universe" and relationships are 99% lazy-loaded - except if you explicitly load them eager.. So this was the "go to strategy" in Laravel - and therefore i am quite familiar with it.. I know, lazy loading has several downsides, for example, it may result in a lot of extra queries (e.g., for complex, deeply nested structures) which may be easier to get / load if you query it eagerly (e.g., everything is get within one single but more complex query)

Thanks for linking to the other issue!