Entity Framework ExpressionAttribute [Mapping]

(from the official guide)

[namespace: Serenity.Data.Mapping] - [assembly: Serenity.Data]

This attribute is used to specify expression of a non-basic field, e.g. one that doesn't actually exist in database.

There can be several types of such fields.

One example is a Fullname field with a calculated expression like (T0.[Firstname] + ' ' + T0.[Lastname]).

public class CustomerRow : Row
{
   public string Firstname
   {
      get { return Fields.Firstname[this]; }
      set { Fields.Firstname[this] = value; }
   }

   public string Lastname
   {
      get { return Fields.Lastname[this]; }
      set { Fields.Lastname[this] = value; }
   }

   [Expression("(T0.[Firstname] + ' ' + T0.[Lastname])")]
   public string Fullname
   {
      get { return Fields.Fullname[this]; }
      set { Fields.Fullname[this] = value; }
   }
}

Be careful with "+" operator here as it is Sql Server specific. If you want to target multiple databases, you should write the expression as: CONCAT(T0.[Firstname], CONCAT(' ', T0.[Lastname]))

Firstname and Lastname are table fields (actual fields in the table), but even if they don't have an expression attribute, they have basic, implicitly defined expressions, T0.Firstname and T0.Lastname (main table is assigned T0 alias in Serenity queries).

In this document, when we talk about a Table Field, it means a field that actually corresponds to a column in database table.

View Field means a field with a calculated expression or a field that originates from another table, like fields that comes from joins in SQL views.

We wrote Fullname expression using T0 alias before the fields that we reference.

It would probably work without that prefix too. But it is better to use it. When you start to add joins, it is possible to have more than one field with same name and experience ambiguous column errors.