Cannot return null for non-nullable field

[theprantadutta]

Describe the bug
I am trying to make a Graphql server with Mikro-ORM. Basically, I have a one-to-many relationship with category and product. Each product should belong to a category and a category can have multiple products. Now I am trying to fetch all the products with their categories and I am having the following error.

Stack trace

    "message": "Cannot return null for non-nullable field Category.title.",
      "locations": [
        {
          "line": 11,
          "column": 7
        }
      ],
      "path": [
        "products",
        0,
        "category",
        "title"
      ],
      "extensions": {
        "code": "INTERNAL_SERVER_ERROR",
        "exception": {
          "stacktrace": [
            "Error: Cannot return null for non-nullable field Category.title.",
...

To Reproduce
Steps to reproduce the behavior:

1. My Product.ts file

      import {
      Entity,
      IdentifiedReference,
      ManyToOne,
      PrimaryKey,
      Property
    } from '@mikro-orm/core'
    import { Field, Float, ObjectType } from 'type-graphql'
    import { ProductArguments } from '../util/types'
    import { Category } from './Category'
    
    @Entity()
    @ObjectType()
    export class Product {
      constructor({ title, price, description }: ProductArguments) {
        this.title = title
        this.price = price
        this.description = description
      }
    
      @Field()
      @PrimaryKey()
      id!: number
    
      @Field()
      @Property()
      title!: string
    
      @Field()
      @Property()
      imageUrl!: string
    
      @Field(() => Float)
      @Property({ type: 'decimal' })
      price!: number
    
      @Field(() => String)
      @Property({ type: 'text' })
      description!: string
    
      @Field(() => Category)
      @ManyToOne(() => Category)
      category!: IdentifiedReference<Category>
    
      @Field()
      @Property()
      createdAt: Date = new Date()
    
      @Field()
      @Property({ onUpdate: () => new Date() })
      updatedAt: Date = new Date()
    }

2. My Category.ts file

    import {
    Cascade,
      Collection,
      Entity,
      LoadStrategy,
      OneToMany,
      PrimaryKey,
      Property
    } from '@mikro-orm/core'
    import { Field, ObjectType } from 'type-graphql'
    import { Product } from './Product'
    
    @Entity()
    @ObjectType()
    export class Category {
      constructor(title: string) {
        this.title = title
      }
    
      @Field()
      @PrimaryKey()
      id!: number
    
      @Field()
      @Property()
      title!: string
    
      @Field(() => [Product])
      @OneToMany({
        entity: () => Product,
        mappedBy: (product) => product.category,
        cascade: [Cascade.REMOVE],
        strategy: LoadStrategy.JOINED
      })
      products = new Collection<Product>(this)
    
      @Field()
      @Property()
      createdAt: Date = new Date()
    
      @Field()
      @Property({ onUpdate: () => new Date() })
      updatedAt: Date = new Date()
    }

3. Now in my resolvers, I am trying to get all the categories with the following.

   const categories = await em
      .createQueryBuilder(Category)
      .select('*')
      .getResult()
    return em.populate(categories, ['products'])

This works perfectly fine. No errors.
But when I try to get all products with their categories like the following:

   const products = await em
      .createQueryBuilder(Product)
      .select('*')
      .getResult()
    return em.populate(products, ['category'])

Or with left join

   return em
      .createQueryBuilder(Product, 'p')
      .select('*')
      .leftJoinAndSelect('p.category', 'c')
      .getResultList()

I got the error described above.
Here's the generated query.

select "p".*, "c"."id" as "c__id", "c"."title" as "c__title", "c"."created_at" as "c__created_at", "c"."updated_at" as "c__updated_at" from "product" as "p" left join "category" as "c" on "p"."category_id" = "c"."id"```

I also tried to run this query in the database and I got the actual result that I wanted. But in GraphQL Server, I got the above error. What am I doing wrong here?

Expected behavior
I want it return my products like the following:

data: {
  products: [
    {
      id: 1,
      title: 'T-shirts',
      ...other staff,
      categories: {
        id
        title
        ...other_stuff
      }
    }
  ]
}

Additional context
You can check out my code for getting all categories too. That's working fine. But I can't get my products.

Versions

Dependency	Version
node	16.1
typescript	4.3
mikro-orm	next(5.x)
your-driver	postgresql

[B4nan]

This is not comming from MikroORM, if you want to report bugs in here, please don't use GQL for reproductions so it is easier to understand.

Looking at your code, I found one issue in the entity definition. If you do not use ts-morph metadata provider, you need to adjust this one:

      @Field(() => Category)
      @ManyToOne(() => Category, { wrappedReference: true }) // added wrappedReference 
      category!: IdentifiedReference<Category>

As with the default metadata provider this information needs to be passed explicitly.

Other ideas:

• ensure you actually have the column as non nullable and it has the data for rows you select
• try logging what the qb.getResult() gives you (maybe after you call em.populate()
• if you don't have any specific reason to use QB, I would rather use EM directly, it's less verbose

[theprantadutta]

@B4nan Thank you for your help. Sorry, if this was a noob mistake. But I just started working on mikro-orm. Anyway, can you provide the query with EM to get all the products with categories? I can't find the proper query in the documentation. The query I found, it always executes two SQL queries. And I am using metadataProvider: TsMorphMetadataProvider in my mikro-orm.config.ts file. Also, I console logged qb.results(), I got all the products and the category actually contains Reference { entity: [Ref<Category>] } .

This is a mikro-orm problem. The problem here is that I am using IdentifiedReference for my Category model with my Product model which actually provides some map methods to get the property like getProperty and others. When I removed the IdentifiedReference<Category> with Category , it works fine. What I didn't understand is that I can't use IdentifiedReference to get the data in graphql.

Anyway, can you provide the actual em query for getting all the products with categories using a single SQL query? I am getting the result I want with em, but it executes two sql query. When I use queryBuilder, I get the result I want using a single SQL query. I can't do it with EM. Thanks btw.

[B4nan]

I got all the products and the category actually contains Reference { entity: [Ref] } .

This means the category is not populated, otherwise it would say Reference { entity: [Category] } without the Ref<...> - so it won't be part of serialized response (only it's PK, not as an object).

This is a mikro-orm problem.

Then provide MikroORM reproduction, without GQL. Reproductions need to be minimal.

When I removed the IdentifiedReference with Category , it works fine.

That sounds exactly as if you would be missing the wrappedReference: true. Have you actually tried to put it there?

Also be sure to update to latest version. I see you said you are using next(5.x), but the whole point of the versions in issue template is to say the exact version you are on, not latest or next, or a version range, that is all wrong - it should be things like 5.0.0-dev.123. Things are moving every day :] I was actually fixing something with identified references few days ago.

Anyway, can you provide the actual em query for getting all the products with categories?

// this:
const products = await em
    .createQueryBuilder(Product)
    .select('*')
    .getResult()
return em.populate(products, ['category'])

// is the same as this:
const products = await em.find(Product, {}, {
  populate: ['category'],
})

[theprantadutta]

Everything else is fine. But

const products = await em.find(Product, {}, {
populate: ['category'],
})

I actually did do this. But it executes two queries. But I want it to generate only one. That's the reason I even considered doing this with the querybuilder.

[B4nan]

const products = await em.find(Product, {}, {
  populate: ['category'],
  strategy: LoadStrategy.JOINED,
})

[theprantadutta]

@B4nan thanks a lot, that's exactly what I wanted.