C#, What's the proper syntax for calling an Expression<Func<T-in, T-out>> from within another lambda expression? - entity-framework-5

Question

I'm working in Entity Framework 5 and having problems creating an expression to use inside a method.
I believe the problem is that normally I would call the expression in a lambda expression such as dbContext.Counties.Select(GetLargeCities()), but in the code I am working with, I am projecting the Counties entity into a view model called CountyWithCities. Where I would normally call the expression, I have a singleton c and don't know how to call the expression there.
The reason I want to accomplish this using an expression is because I want the GetCountiesWithCities method to hit the database once, with Entity Framework constructing a complex graph for all the objects in the result.
For some reason the code below is producing the error `The name 'GetLargeCities' does not exist in the current context."
public class CountyWithCities // this is a view model
{
public int CountyID { get; set; }
public string CountyName { get; set; }
public IEnumerable<City> Cities { get; set; }
}
public class City // this is an entity
{
public int CityID { get; set; }
public string CityName { get; set; }
public int Population { get; set; }
}
public IEnumerable<CountyWithCities> GetCountiesWithCities(int StateID)
{
return dbContext.States
.Where(s => s.StateID = StateID)
.Select(s => s.Counties)
.Select(c => new CountyWithCities
{
CountyID = c.CountyID,
CountyName = c.CountyName,
Cities = GetLargeCities(c) // How do I call the expression here?
});
}
public Expression<Func<County, IEnumerable<City>>> GetLargeCities = county =>
county.Cities.Where(city => city.Population > 50000);
Thanks!

Answer 1

I normally do this with an extension method.
public static IQueriable<City> LargeCities(this IQueriable<County> counties){
return counties
.SelectMany(county=>county.Cities.Where(c=>c.Population > 50000));
}
usage:
dbContext.Counties.LargeCities()
public IEnumerable<CountyWithCities> GetCountiesWithCities(int StateID)
{
return dbContext.States
.Where(s => s.StateID = StateID)
.Select(s => s.Counties)
.LargeCities()
.GroupBy(c=>c.County)
.Select(c => new CountyWithCities
{
CountyID = g.Key.CountyID,
CountyName = g.Key.CountyName,
Cities = g.AsQueriable() // i cant remember the exact syntax here but you need the list from the group
});
}