How to get data from two tables and save it in DTO in Net Core Entity Framework?

Question

I just explain this in example, say I have two tables, field and modulefield

field
id     name          ismodule
1      Firstname     1
2      Lastname      0

modulefield
id     moduleid      fieldid
1      22            1

That is a simple representation of my database. Now in EF, how can I get a row with these columns

modulefield.moduleid     field.id     field.name
22                       1            Firstname

Basically, I just want to get the modulefield.moduleid if the field.ismodule is true.

The equivalent sql script I think for this is

SELECT b.submoduleid, a.* FROM field a, modulefield b;

My current code is this(snippet)

   var result = new List<FieldDTO>();
   var fields = await _repo.GetAsync<Field>();
   result = _mapper.Map<List<FieldDTO>>(fields);

Answer 1

I assume there could be more then one Module per Field. Query:

SELECT mf.submoduleid, f.id, f.name 
FROM field AS f, modulefield AS mf
WHERE f.ismodule = 1 AND
    f.id = mf.fieldid

EF Linq expression:

List<FieldDTO> result = _context.Fields
    .Include("ModuleField")
    .Where(f => f.ismodule == true)
    .Select(f => new FieldDTO(){
        ModuleFieldModuleId = f.ModuleField.moduleid,
        FieldId = f.id,
        FieldName = f.Name})
    .ToList();

ModuleField is navigation property in Field class.

EDIT:
Models:

public class Field
{
    [Key]
    public long Id { get; set; }

    public string FieldName { get; set; }

    [NotMapped]
    public bool IsModule 
    {
        get
        {
            return this.Module != null;
        }
    }

    public virtual Module Module {get; set;}
}

public class Module
{
    [Key]
    public long Id { get; set; }

    public string ModuleName { get; set; }

    public virtual IEnumerable<Field> Fields { get; set }
}

On model creating:

modelBuilder.Entity<Field>()
                .HasOptional(s => s.Module); // Marking property optional

Getting the Module.ModuleId if IsModule of Field is true:

List<FieldDTO> result = _context.Fields
    .Include("Module")
    .Where(f => f.IsModule == true)
    .Select(f => new FieldDTO(){
        ModuleFieldModuleId = f.Module.Id,
        FieldId = f.Id,
        FieldName = f.Name})
    .ToList();

Answer 2

You could use linq join to return a List<FieldDTO> directly.

var result = (from e in _context.Fields
                     join mf in _context.ModuleFields on e.Id equals mf.FieldId
                     where e.IsModule == 1

                     select new FieldDTO
                     {
                         ModulefieldModuleid = mf.ModuleId,
                         FieldId = e.Id,
                         FieldName=e.Name


                     }).ToList();