How to load different children classes to parent collection in Entity Framework Core? - asp.net-core

Question

Given the following:
public class Parent
{
public ChildType childType;
}
public class ChildA : Parent { ... }
public class ChildB : Parent { ... }
public enum ChildType {
childA,
childB
}
public class Content {
public long contentId;
public string? name;
public ICollection<Parent>? contentCollection; <--
...
}
I would like to use the Content class as part of an API. Is it possible to load both children into the collection just using the enum as a discriminator to determine which to cast to?
My understanding is the child objects would need to be loaded from EF as their child class first, then cast to the parent class before being added to the collection as they would be missing properties upon casting back to the child class otherwise. Is this correct? And how can the dbContext be configured to handle this when accessing through the Content class?
Apologies for all the questions, I have not done this before and cannot find an example online. I would like to know any thoughts, pointers or general info before proceeding. Please say if anything is unclear or more info is required.
Edit:
I was trying to map the child objects as their types from the DB, upcast to the parent type to be able to add multiple types to the one collection and then downcast when required for use. As far as I was aware, EF did not have the functionality to do this.

Answer 1

For anyone else who comes across this which needs assistance, I solved my issue by just using ADO.NET which is what entity framework is built around. I was getting stuck by trying to get this working using EF but my belief is it is not able to be done with EF.
Formatting is off a little, and I have renamed everything to suit my original question but here is the solution involved:
Writing an SP to retrieve the data similar to if the objects were stored in a TPH pattern.
Calling that SP using SQLConnection/SQLCommand. (I added this into my context class to keep DAL together but unsure if this is best practice)
public async Task<Collection<Parent>> GetModelMapCollectionAsync(long id) {
Collection<Parent> parentCollection;
using (SqlConnection connection = new SqlConnection(this.Database.GetConnectionString()))
{
using (SqlCommand sqlCommand = new SqlCommand("GetModelMapCollectionAsync", connection))
{
sqlCommand.CommandType = System.Data.CommandType.StoredProcedure;
sqlCommand.Parameters.Add(new SqlParameter("#id", id));
await connection.OpenAsync();
await sqlCommand.ExecuteNonQueryAsync();
using (SqlDataReader sqlDataReader = await sqlCommand.ExecuteReaderAsync())
{
MapCollectionResult(sqlDataReader, out parentCollection);
}
}
}
return parentCollection; }
Using a nuget package called Dapper, create row parsers for each type (easiest solution for readability/simplicity IMO)
Use the discriminator column to determine which parser to use on each row returned from the SP. This creates the child object from the row which allows it to be downcast back later.
Add that parsed object to the collection.
private void MapCollectionResult(SqlDataReader sqlDataReader, out Collection parentCollection)
{
parentCollection= new Collection();
var parentParser = sqlDataReader.GetRowParser<Parent>(typeof(Parent));
var paramClassParser = sqlDataReader.GetRowParser<ParamClass>(typeof(ParamClass));
var childAParser = sqlDataReader.GetRowParser<ChildA>(typeof(ChildA));
var childBParser = sqlDataReader.GetRowParser<ChildB>(typeof(ChildB));
ChildType type = ChildType.None;
Parent parent;
while (sqlDataReader.Read())
{
type = (ChildType)sqlDataReader["ChildTypeId"];
switch(type)
{
case ChildType.ChildA:
parent = childAParser(sqlDataReader);
break;
case ChildType.ChildB:
parent = childBParser(sqlDataReader);
break;
default:
parent = parentParser(sqlDataReader);
break;
}
parent.paramClass = paramClassParser(sqlDataReader);
parentCollection.Add(parent);
}}