c# - LINQ: Fill objects from left join

Question

I'm new to linq. How do I load my objects using LINQ from a left join database query (PostGIS).

This is my database query:

SELECT          
                dt.id, 

                dt.type, 
                dta.id as "AttId",
                dta.label,
                dtav.id as "AttValueId",
                dtav.value

FROM            public."dataTypes" dt, 
                public."dataTypeAttributes" dta
LEFT JOIN       public."dataTypeAttributeValues" dtav
ON              dta.id = "dataTypeAttributeId"        
WHERE           dt.id = dta."dataTypeId"
ORDER BY        dt.type, dta.label, dtav.value

And here is example output:

I have 3 entities:

public class TypeData
{
    public int ID { get; set; }
    public string Type { get; set; }
    public TypeDataAttribute[] Attributes { get; set; }

}
public class TypeDataAttribute
{
    public int ID { get; set; }
    public string Label { get; set; }
    public TypeDataAttributeValue[] Values { get; set; }
}

public class TypeDataAttributeValue
{
    public int ID { get; set; }
    public string Value { get; set; }
}

Update

Here is where I get stuck:

...
using (NpgsqlDataReader reader = command.ExecuteReader())
{

    if (reader.HasRows)
    {
        IEnumerable<TypeData> typeData = reader.Cast<System.Data.Common.DbDataRecord>()
            .GroupJoin( //<--stuck here.
    }
...

SOLUTION:

using AmyB's answer, this is what filled my objects:

using (NpgsqlDataReader reader = command.ExecuteReader())
{

    if (reader.HasRows)
    {

        var groups = reader.Cast<System.Data.Common.DbDataRecord>()
            .GroupBy(dr => new { ID = (int)dr["id"], AttID = (int)dr["AttId"] })
            .GroupBy(g => g.Key.ID);

        typeDataList = (
            from typeGroup in groups
            let typeRow = typeGroup.First().First()
            select new TypeData()
            {
                ID = (int) typeRow["id"],
                Type = (string) typeRow["type"],
                Attributes = 
                (
                    from attGroup in typeGroup
                    let attRow = attGroup.First()
                    select new TypeDataAttribute()
                    {
                        ID = (int)attRow["AttId"],
                        Label = (string)attRow["label"],
                        PossibleValues =
                        (
                            from row in attGroup
                            where !DBNull.Value.Equals(attRow["AttValueId"])
                            select new TypeDataAttributeValue() { ID = (int)row["AttValueId"], Value = (string)row["value"] }
                        ).ToArray()
                    }
                ).ToArray()
            }
        );
    }
}

Answer 1

So - if I understand right - you have a database query that you are happy with, but you want to take the row-column shaped result and project it into a hierarchically shaped result.

---

Suppose the results are in a List<Row>

public class Row
{
  public int id {get;set;}
  public string type {get;set;}
  public int attid {get;set;}
  public string label {get;set;}
  public int? attvalueid {get;set;}
  public string value {get;set;}
}

Then you would group twice, and turn each top-level group into a Type, each child-level group into an Attribute and each row into a Value (if the row is not an empty value).

List<Row> queryResult = GetQueryResult();

//make our hierarchies.
var groups = queryResult
  .GroupBy(row => new {row.id, row.attid})
  .GroupBy(g => g.Key.id);

//now shape each level
List<Type> answer =
(
  from typeGroup in groups
  let typeRow = typeGroup.First().First()
  select new Type()
  {
    id = typeRow.id,
    type = typeRow.type,
    Attributes =
    (
      from attGroup in typeGroup
      let attRow = attGroup.First()
      select new Attribute()
      {
        id = attRow.attid,
        label = attRow.label
        Values =
        (
          from row in attRow
          where row.attvalueid.HasValue  //if no values, then we get an empty array
          select new Value() {id = row.attvalueid, value = row.value }
        ).ToArray()
      }
    ).ToArray()
  }
).ToList();