C# linq 从子集合中选择项目
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linq select items from child collection
提问by GuestMVCAsync
Below are my classes. I have a product that contains list of days. Each day has a city property.
下面是我的课。我有一个包含天数列表的产品。每天都有一个城市属性。
I need to create a linq query that will give me the distinct cities that are used on all my products in the system.
我需要创建一个 linq 查询，该查询将为我提供系统中所有产品使用的不同城市。
I tried something like this but it does not work:
我试过这样的事情，但它不起作用：
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; }
}
采纳答案by SLaks
You need to call the SelectManyfunction, which takes a single item and lets you get multiple items from it.
您需要调用该SelectMany函数，该函数接受单个项目并让您从中获取多个项目。
For example:
例如：
var cities = NHibernateSession.Linq<Product>()
.SelectMany(p => p.Days)
.Select(p => p.City)
.Where(c => c != null)
.Distinct();
Note that if the Cityclass doesn't implement Equalsand GetHashCodecorrectly, this will return duplicates.
请注意，如果City类没有实现Equals和GetHashCode正确的，这将返回重复。
You can do this using query comprehension syntax like this: (Untested)
您可以使用这样的查询理解语法来做到这一点：（未经测试）
var cities = (from product in NHibernateSession.Linq<Product>()
from day in product.Days
where day.City != null
select day).Distinct();
