I am working with two promises and have simplified this to make things easier to look at.

Note Cars and ServiceRecords are 2 different tables.

I would like my end dataset to look like this:

car = {
make: Honda,
color:red,
id:12,
serviceRecords:[
      {type: oil change,
       customerHappy: Yes,
       id:1
       carid:12
       },
      {type: oil change,
       customerHappy: no
       id:2
       carid:12
       }
     ]
}

First Promise

getCar() {
    return new Promise((resolve, reject) => {
      const id = this.route.snapshot.paramMap.get('id');
      // find the car we want
      const car = Xrm.WebApi.retrieveRecord('car', `${id}`);
      if (car ) {
        resolve(car ); // send the car
      } else {
        reject(Error('No Car Found!'));
      }
    });
  }

Second Promise

hydrateCar(car) {
    return new Promise((resolve, reject) => {
      const services:any = Xrm.WebApi.retrieveMultipleRecords('servicerecord', `?$filter=carid eq ${car.id}`).then((results) => {
        for (let i = 0; i < results.entities.length; i++) {
          const result = results.entities[i];
           car.serviceRecords=result;
        }
        resolve(car);
      });
    });
  }

Call the function

this.getCar().then((car) => {
      console.log('car', car);
      return this.hydrateCar(car);
    })
      .catch((error) => {
        console.log(error);
      });
    this.loading = false;
  }

All I get back is the last iteration of the service record. I would like the key car.ServiceRecords to be an array of all the records.

>Solution :

hydrateCar(car) {
  return new Promise((resolve, reject) => {
    Xrm.WebApi.retrieveMultipleRecords('servicerecord', `?$filter=carid eq ${car.id}`).then((results) => {
      const serviceRecords = []; 
      
      for (let i = 0; i < results.entities.length; i++) {
        const result = results.entities[i];
        serviceRecords.push(result); 
      }
      
      car.serviceRecords = serviceRecords;
      resolve(car);
    });
  });
}

The issue you’re facing is that you’re overwriting the car.serviceRecords property in each iteration of your loop in the hydrateCar function. Instead, you should accumulate the service records into an array.