Cannot read properties of undefined (reading 'concat')

Question

Roles entity has many-many relation with entitlement entity as shown below:

     @ManyToMany(() => Entitlement)
          @JoinTable({
            name: 'role-entitlements',
        joinColumn: {
          name: 'role_id',
          referencedColumnName: 'id',
        },
            inverseJoinColumn: {
              name: 'entitlement_id',
              referencedColumnName: 'id',
            },
          })
          entitlements: Entitlement[];
    

Below given is the function to add multiple entitlements to a Role:

     async linkEntitlement(id: string, linkEntitlementDto: LinkEntitlementDto) {
        const role: Role = await this.findOne(id);

          if (!role) {
            throw new CustomException('role not found');
        }

        const entitlement: Entitlement = await this.entitlementService.findOne(
          linkEntitlementDto.entitlementId,
        );
        if (!entitlement) {
          throw new CustomException('entitlement not found');
        }

        const entitlements: Entitlement[] = role.entitlements;
        role.entitlements = entitlements.concat(entitlement);
        await this.rolesRepository.save(role);
      }

Below given is the json content I am trying to post via Postman:

    {
        "entitlements":[
            {"id":"12d7b37e-1464-4ffa-b9af-779ab298afb9"}

        ]
    }

I am getting the error as mentioned above What I am trying to do here is there's an Role entity and an entitlement entity. I created many- many relationship between role and entitlement. I created a function LinkEntitlement where I can map role & entitlement. I am posting the data via postman here but I am facing such an error. What is it that I can change in this code?below is the postman image

Answer 1

entitlements is undefined because you didn't fetch the relationship with Entitlement and it is lazily loaded by default.

To do so:

const role = await this.findOne(id, { relations: ['entitlements'] });

btw I advise you to mark that field entitlements: Entitlement[] as optional. Then you won't get confused on why TypeScript is telling you that something is not undefined when it is at runtime.