Not sure how to do a "join" in @ngrx/data

Question

I have three entities

evidence {claimId; string, 
          subject1Id: string, 
          subject2Id:string, 
          claim: Claim, 
          subject1: Subject, 
          subject2: Subject
         }
claim {id, ...}
subject {id, ...}

I am processing a stream of each. Assuming claimId, subject1Id and subject2Id are populated AND that claim, subject1, and subject2 are not and I want to end up with a new collection:

evidence { claimId, 
           subject1Id, 
           subject2Id, 
           claim: <where claim.id == this.claimId>,
           subject1: <where subject.id == this.subject1Id>,
           subject2: <where subject.id == this.subject2Id>
         }

I just can't seem to get my head around how to build the store pipe.

Suggestions or a pointer to examples much appreciated.

Answer 1

just combine your streams and unite in in a model that you want

export class EvidencesService extends EntityCollectionServiceBase<Evidence> {
  ....
  constructor(..., pricate subjectsService: SubjectsService,  private claimsServiceL ClaimsService) {
  ...
  }

  expandedEntities$ = combineLatest([
     this.entities$,
     this.subjectsService.entities$,
     this.claimsService.entities$,
  ]).pipe(map(([evidences, subjects, claims]) =>
    Object.fromEntries(Object.entries(evidences)).map(([id, evidence]) => [id, {
      ...evidence,
      claim: claims[evidence.claimId],
      subject1: subjects[evidence.subject1Id],
      subject2: subjects[evidence.subject2Id],
    }])
    
  ));
}