linq select items from child collection

Question

Below are my classes. I have a product that contains list of days. Each day has a city property.

I need to create a linq query that will give me the distinct cities that are used on all my products in the system.

I tried something like this but it does not work:

var cities = from product in NHibernateSession.Linq<Product>() select new { city = product.Days.Where(d => d.City != null).Distinct() }; //This returns the day items but i need distinct cities

   public class Product : EntityBase
   {
        public virtual string Name { get; set; }
        public virtual IList<ProductDayDefinition> Days { get; set; }
   }

   public class ProductDayDefinition : EntityBase
   {
        public virtual Product Product { get; set; }
        public virtual City City { get; set; }
   }

Answer 1

You need to call the SelectMany function, which takes a single item and lets you get multiple items from it.

For example:

var cities = NHibernateSession.Linq<Product>()
                .SelectMany(p => p.Days)
                .Select(p => p.City)
                .Where(c => c != null)
                .Distinct();  

Note that if the City class doesn't implement Equals and GetHashCode correctly, this will return duplicates.

You can do this using query comprehension syntax like this: (Untested)

var cities = (from product in NHibernateSession.Linq<Product>() 
              from day in product.Days
              where day.City != null
              select day).Distinct();