C# Linq find particular item in nested array object

Question

I am developing an application using asp.net core webapi and azure cosmosdb. I need to find an item from the list of objects. I am not experienced with linq. In the below json, i need to find a sector id who got a learnerId": "123". I am expecting output like:

"id": "7d65fa4c-86ed-4e07-9e9a-a1583df5715c"

Sector:

[
  {
    "name": "Emp1",
    "edsNumber": "",
    "sectors": [
      {
        "id": "7d65fa4c-86ed-4e07-9e9a-a1583df5715c",
        "name": "Technology1",
        "learners": [
          {
            "learnerId": "123",
            "learnerName": "Learner1",
            "dateAdded": 1612872051
          },
          {
            "learnerId": "456",
            "learnerName": "Learner2",
            "dateAdded": 1612877369
          },
          {
            "learnerId": "789",
            "learnerName": "Learner3",
            "dateAdded": 1612877372
          },
          {
            "learnerId": "12314",
            "learnerName": "Learner4",
            "dateAdded": 1613065233
          },
          {
            "learnerId": "2346",
            "learnerName": "Learner5",
            "dateAdded": 1613577350
          }
        ],
        "course": {
          "name": "Angularv10",
          "isCompleted": true
        },
        "contact": {
          "name": "Voytek",
          "jobTitle": "Designer1",
          "email": "[email protected]",
          "isCompleted": true
        }
      }
    ]
  },
  {
    "name": "Emp2",
    "edsNumber": "",
    "sectors": [
      {
        "id": "7d65fa4c-86ed-4e07-9e9a-a1583df5235c",
        "name": "Technology1",
        "learners": [
          {
            "learnerId": "834457",
            "learnerName": "Learner6",
            "dateAdded": 1612872051
          },
          {
            "learnerId": "11122",
            "learnerName": "Learner7",
            "dateAdded": 1612877369
          }
        ],
        "course": {
          "name": "Angularv10",
          "isCompleted": true
        },
        "contact": {
          "name": "Voytek",
          "jobTitle": "Designer1",
          "email": "[email protected]",
          "isCompleted": true
        }
      }
    ]
  }
]

I tried the following:

        var sectorId = (from e in employers
                      from s in e.Sectors
                      from l in s.Learners
                      where l.LearnerId == learnerEntity.Id
                      select s.Id).FirstOrDefault();

Is there a better way to achieve? And how to get both the SectorId and the employer the sector belongs to?

Answer 1

LINQ query that looks for a single item that satisfies a condition on a child object would have two components:

• A call to SingleOrDefault (or Single if the item you look for must be there), and
• A call to Any nested inside the first condition to see that the child item is there.

Since your sectors are embedded inside employers, you could flatten the list with SelectMany.

Here is how it would look:

var sectorId = employers
    .SelectMany(e => e.Sectors) // Flatten sectors
    .SingleOrDefault(
        s => s.Learners.Any(sl => sl.Id == targetId)
    ).Id;