Entity Framework NotMapped Join

Question

I have a model that links to a DB table with a few non-mapped fields that I need to populate via a join in EF.

Here is my current attempt at this which returns the following error:

The specified type member db_current_value is not supported in LINQ to Entities. Only initializers, entity members, and entity navigation properties are supported.

Model = MainTable
composite_mapped_id1
composite_mapped_id2
general_mapped_label
mapped_calucluation
not_mapped_property_current //not_mapped
not_mapped_property_previous //not_mapped


var queryCurrent = from t in _context.MainTable
                               where t.Data_Period == "04/01/2021"
                               select
                               new //Anonymous
                               {
                                   //CASE STATEMENTS HERE
                                   composite_id1 = t.composite_mapped_id1,
                                   composite_id2 = t.composite_mapped_id2,
                                   general_label = t.general_mapped_label,
                                   not_mapped_property_current = t.mapped_calucluation

                               };



            var queryPrevious = from t in _context.MainTable
                                where t.Data_Period == "01/01/2021"
                                select
                               new //Anonymous
                               {
                                   composite_id1 = t.composite_mapped_id1,
                                   composite_id2 = t.composite_mapped_id2,
                                   not_mapped_property_previous = t.mapped_calucluation

                               };



            var queryFinal = (from c in queryCurrent
                              from p in queryPrevious
                              where c.composite_id1 == p.composite_id1 && c.composite_id2 == p.composite_id2
                              select c).AsEnumerable()
                            .Select(x => new MainTable
                            {

                                general_label = t.general_mapped_label,
                                not_mapped_property_current = x.not_mapped_property_current,
                                not_mapped_property_previous =  c.not_mapped_property_previous //NOT SURE HOW TO REFERENCE THIS 


                            }).ToList();

Any suggestions on how to tweak this or perhaps a completely different approach?

UPDATE: I Decided to go with a NonMapped ViewModel to support this. Not sure if it's ideal though:

var current = _context.MainTable.Where(c => c.Data_Period == "01/01/2021").ToList();
var previous = _context.MainTable.Where(p => p.Data_Period == "04/01/2021").ToList();
    
var queryFinal = (from c in current
                  from p in previous
                  where c.composite_id1 == p.composite_id1 && 
                  c.composite_id2 == p.composite_id2
                  select
                  new MainTable_Interface_Model
                  {
                        general_label = c.general_mapped_label,
                        property_current = c.mapped_calucluation,
                        property_previous = p.mapped_calucluation
    
                  }).ToList();

UPDATE #2 First UPDATE was too slow so for now I went with my comfort zone of SQL

_context.Database.SqlQuery<MainTable_Interface_Model>(strSQL).ToList<MainTable_Interface_Model>();