How to fill data in nested Lists based on Ids in C#?

public class CarDTO
{
    public int CarId { get; set; }
    public string CarName { get; set; }
    public List<PolicySummaryDTO> PolicySummaries { get; set; } 
}

public class PolicySummaryDTO
{
    public long PolicyId { get; set; }
    public string PolicyName { get; set; }
    public decimal Amount { get; set; }
}

I have List<CarDTO> Cars each car has already list of policies List<PolicySummaryDTO> PolicySummaries. PolicyId is filled. But other properties like PolicyName, Amount, etc. are not. The thing I need to do is get data from DB _contex.PolicySummary and I need to complete missing fields.

I know that I could do this in this way.

1. Get all PolicyIds var policyIds = cars.SelectMany(t => t.PolicySummaries).Select(r => r.PolicyId);

2. Get PolicyData based on Ids var policyData = _context.PolicySummary.Where(t => policyIds.Contains(t.PolicyId));

3. And then using foreach I can fill data.

   foreach (var car in cars)
   {
       foreach (var policy in car.PolicySummaries)
       {
           var policyDataToUse = policyData.Single(t => t.PolicyId == policy.PolicyId);
           policy.Amount = policyDataToUse.Amount;
           policy.PolicyName = policyDataToUse.PolicyName;
       }
   }
   

Everthing will works fine, but I wondering whether I can do it in more elegant way, maybe using somehow LINQ JOIN or something, or maybe my solution is totally correct?

EDIT - solution with dictionary

var policyIds = cars.SelectMany(t => t.PolicySummaries).Select(r => r.PolicyId);
var policyDict = _context.PolicySummary.Where(t => policyIds.Contains(t.PolicyId)).ToDictionary(t => t.PolicyId);

foreach (var car in cars)
{
    foreach (var policy in car.PolicySummaries)
    {
        var policyDataToUse = policyDict[policy.PolicyId];
        policy.Amount = policyDataToUse.Amount;
        policy.PolicyName = policyDataToUse.PolicyName;
    }
}