Question

I have repositories (e.g. ContactRepository, UserRepository and so forth) which encapsulate data access to the domain model.

When I was looking at searching for data, e.g.

• finding a contact whose first name
  starts with XYZ

• a contact whose birthday is after
  1960

  (etc),

I started implementing repository methods such as FirstNameStartsWith(string prefix) and YoungerThanBirthYear(int year), basically following the many examples out there.

Then I hit a problem – what if I have to combine multiple searches? Each of my repository search methods, such as above, only return a finite set of actual domain objects. In search for a better way, I started writing extension methods on IQueryable<T>, e.g. this:

public static IQueryable<Contact> FirstNameStartsWith(
               this IQueryable<Contact> contacts, String prefix)
{
    return contacts.Where(
        contact => contact.FirstName.StartsWith(prefix));
}        

Now I can do things such as

ContactRepository.GetAll().FirstNameStartsWith("tex").YoungerThanBirthYear(1960);

However, I found myself writing extension methods (and inventing crazy classes such as ContactsQueryableExtensions all over, and I lose the “nice grouping” by having everything in the appropriate repository.

Is this really the way to do it, or is there a better way to achieve the same goal?

Answer 1

@Alex – i know this is an old question, but what I would be doing would be letting the Repository do really simple stuff only. This means, get all records for a table or view.

Then, in the SERVICES layer (you are using an n-tiered solution, right? 🙂 ) i would be handling all the ‘special’ query stuff there.

Ok, example time.

Repository Layer

ContactRepository.cs

public IQueryable<Contact> GetContacts()
{
    return (from q in SqlContext.Contacts
            select q).AsQueryable();
}

Nice and simple. SqlContext is the instance of your EF Context .. which has an Entity on it called Contacts .. which is basically your sql Contacts class.

This means, that method basically is doing: SELECT * FROM CONTACTS … but it’s not hitting the database with that query .. it’s only a query right now.

Ok .. next layer.. KICK … up we go (Inception anyone?)

Services Layer

ContactService.cs

public  ICollection<Contact> FindContacts(string name)
{
    return FindContacts(name, null)
}

public ICollection<Contact> FindContacts(string name, int? year)
{
   IQueryable<Contact> query = _contactRepository.GetContacts();
   
   if (!string.IsNullOrEmpty(name))
   {
       query = from q in query
               where q.FirstName.StartsWith(name)
               select q;
   }

   if (int.HasValue)
   {
       query = from q in query
               where q.Birthday.Year <= year.Value
               select q);
    }

    return (from q in query
            select q).ToList();
}

Done.

So lets recap. First, we start our with a simple ‘Get everything from contacts‘ query. Now, if we have a name provided, lets add a filter to filter all contacts by name. Next, if we have a year provided, then we filter the birthday by Year. Etc. Finally, we then hit the DB (with this modified query) and see what results we get back.

NOTES:-

• I’ve omitted any Dependency Injection for simplicity. It’s more than highly recommended.
• This is all pseduo-code. Untested (against a compiler) but you get the idea ….

Takeaway points

• The Services layer handles all the smarts. That is where you decide what data you require.
• The Repository is a simple SELECT * FROM TABLE or a simple INSERT/UPDATE into TABLE.

Good luck 🙂