Error: 'ParentTableName' is not a member of type 'EntityModel.ChildTableName' in the currently loaded schemas. Near simple identifier, line 6, column 4.

•  MAKE sure tables are related  (primary - foreign keys are set)
•  Note that 

it.ParentTable.Id= @SomeId - tells db to load only items that have that @SomeId

•  MAKE sure name in where parameters is correct (usually the same as QueryStringField)

   <WhereParameters>
          <asp:QueryStringParameter Name="SomeId" QueryStringField="SomeId" Type="Int32" />
    </WhereParameters>