How to map SQL Server vertical data into Entity Framework class and then to Angular page

Question

I'm new to this forum, and I have only few experience using Entity Framework.

I hope to post a clear issue: I have already implemented classes, API and SPA page to show stock technical properties. Initially, all the fields were strings or numbers, without any kind of check of the correctness of the values (I have inherited dirty data from an old database). Some of the string fields need to be normalized, reading data from a "clean" list. I tried to extract the different values from the original (dirty) table, in order to create a lookup table.

Due to I have different fields with the same data type, I figured out to develop a vertical table as follows:

    Id  CategoryName    CategoryMake    CategoryType
...
    10  Engine          Mercedes        OM926LA T4i
    11  Engine          Mercedes        OM934LA T4f
    12  Engine          Mercedes        OM936LA T4f
    13  Engine          Perkins         SERIE 700
    14  Engine          Volvo           TAD1183 VE St.5
    15  Engine          Volvo           TAD720 VE
    16  Engine          Volvo           TAD750 VE
    17  Engine          Volvo           TAD870 VE T4f
...
    25  EngineECU       Mercedes        OM906LA.111/2-00
    26  EngineECU       Mercedes        OM926LA.E3B/5-01
    27  EngineECU       Mercedes        OM934LAE4-3-01
    28  EngineECU       Volvo           TAD870 VE T4f
    29  EngineECU       Volvo           TAD871 VE T4f
    30  EngineECU       Volvo           TAD872 VE T4f
    31  FirstAxle       AxleTech        5511SFW8661
    32  FirstAxle       AxleTech        5511SFW8662
    33  FirstAxle       AxleTech        A87213661
    34  FirstAxle       AxleTech        A87213662
    35  FirstAxle       Kessler         D81PL478NLB5340

In the web page, I need to put, for each CategoryName, a list of distinct CategoryMake, and for each CategoryName + CategoryMake, the list of CategoryType, using mat-autocomplete fields.

My issue is that I need to convert all these fields from strings to complex structures containing CategoryName, CategoryMake and CategoryType, but I'm having errors on API retrieval methods.

ADDITIONAL INFO: Each CategoryName represents a block of info (Engine data, First Axle data and so on). Each CategoryMake represents the brand of the piece (the Engine can be Mercedes, Volvo, Perkins, etc.). Each CategoryType represents the model of the piece.

I've tried to follow another structure that actually works, but based on a lookup table with only two fields (code and description). The search key is the code (CatMercCode) and the object that references the table is defined in a CatMercResource class:

public class StockItemResource
{
    public int Id { get; set; }
    public int? StockId { get; set; }
    public string CatMercCode { get; set; }
    public CatMercResource CatMerc { get; set; }
}

In my class I tried this definition for the field called "GeneralType", represented in the lookup table as:

Id  CategoryName    CategoryMake    CategoryType
292 General         Type            
293 General         Type            BC182
294 General         Type            BC182 4x2
295 General         Type            MG5032
296 General         Type            ML1812R
297 General         Type            RT222
298 General         Type            RT223
299 General         Type            RT282
300 General         Type            RT283
301 General         Type            RT323
302 General         Type            RT403
303 General         Type            STACKACE-E7-9
304 General         Type            TT223
305 General         Type            TT223 CC
306 General         Type            TT2516
307 General         Type            YT182
308 General         Type            YT182 4x2
309 General         Type            YT193
310 General         Type            YT203EV
311 General         Type            YT222
312 General         Type            YT222 4X2
313 General         Type            YT223
314 General         Type            YT282

And my class:

public class StockTechnicalInfoResource
{
    public string GeneralName { get; set; }
    public string GeneralMake { get; set; }
    public StockTechnicalInfoTypeResource GeneralType { get; set; }
    //public DateTime? GeneralDeliveryDate { get; set; }
    public double? GeneralWheelBase { get; set; }
    public double? GeneralWeightFront { get; set; }
    public double? GeneralWeightRear { get; set; }

    public string EngineMake { get; set; }
    public StockTechnicalInfoTypeResource EngineType { get; set; }
    public string EngineSerialNumber { get; set; }
    public string EngineWarrantyPeriod { get; set; }

    public string EngineECUMake { get; set; }
    public StockTechnicalInfoTypeResource EngineECUType { get; set; }
    public string EngineECUPartNumber { get; set; }
    public string EngineECUSerialNumber { get; set; }
    public double? GeneralWeightTotal { get; set; }
    ...
}

In any way I'd need to read from the same table columns and put values on different properties. I hope to explain myself, even if I'm afraid that is too complex to be explained in this way...

Could anyone suggest me any idea? Many thanks in advance!

Best regards,

Laura