如何使用Entity Framework对具有不同名称的属性进行通用LINQ查询？

Question

Using Entity Framework 4.0, I've defined two simple Entities with generated code that looks like this:

public partial class Person : EntityObject
{
   public string Name { ... generated code ... }
   public DateTime Birthday { ... generated code ... }
}

public partial class Sale : EntityObject
{
   public int ProductId { ... generated code ... }
   public DateTime DateSold { ... generated code ... }
}

This particular example is contrived, but represents a real problem I'm facing. Often in my code, I want to restrict Entities to ones that appear in a particular date range. So I have a lot of instances of:

entities.Sales.Where(sale => sale.DateSold > startDate && sale.DateSold < endDate);

entities.People.Where(person => person.Birthday > startDate && person.Birthday < endDate);

is it possible to set up a common method that can handle this? Something like

entities.Sales.WithinRange(startDate, endDate); would be perfect, and I realize that I could set up an extension method for each and every IQueryable<T> , but I'd like to flexibility to be able to use the WithinRange() method inside common code. For example:

public static IQueryable<T> GetSortedNonNullObjectsInRange<T>(this IQueryable<T> data, DateTime startDate, DateTime endDate) where T : IHasDateDefined
{
   return data.Where(entity => entity != null).WhereInRange(startDate, endDate).OrderBy(entity => entity.Date);
}

when I try to use a common interface (like IHasDateDefined), I have the Date property on the interface return the Birthday or DateSold as appropriate, but then Entity Framework throws errors that it can't build an expression.

Answer 1

I think you can achieve what you want with the following:

• add a static method on your entity-object with the following signature: Expression> CheckWithinRange (DateTime startDate, DateTime endDate) (and yes, in the Person, the T should actually be replaced with 'Person')

• extend your context using a .tt script so that a method is added for each table (eg Person and Sale), which uses the static added methods to add the expression of the entity-object to the request IQueryable. You can also indeed create an extension method to the specific IQueryable<...> types.

Example.

The method added to the Person class:

Expression<Func<Person, bool>> CheckWithinRange (DateTime startDate, DateTime endDate) {
    return p => startDate <= p.Birthday && p.Birthday <= endDate;
}

The extension method added to IQueryable

public IQueryable<Person> WithinRange(this IQueryable<Person> data, DateTime startDate, DateTime endDate) {
    return data.Where(Person.CheckWithinRange(startDate, endDate));
}

So instead of adding a property to each class which returns the property you want to check, add the logic of the check to each class.