Linq 查询以包括连接集合的项目数

Question

I have three tables in DB

• Person: [PersonId, Name, AddressId]
• Address: [AddressId, CityName]
• UploadPhoto: [PhotoId, PersonId]

Assume one person has one address, but can have multiple uploaded photo. I want to write a Linq query to produce a joined collection with item like

{
   PersonId: 5;
   Name: "Jack";
   CityName: "Edmonton";
   NumOfUploadPhoto: 10;
}

What's the Linq query looks like? Thanks

Answer 1

So you have a one-to-many between Addresses and Persons: Every Address has zero or more Persons, every Person lives at exactly one Address, namely the Address that the foreign key Person.AddressId refers to.

Similarly you have a one-to-many relation between Persons and Photos: every Person has uploaded zero or more Photos, every Photo has been uploaded by the Person that the foreign key Photo.PersonId refers to.

Whenever you wnat "items with their zero or more sub-items", like Schools with their Students, Customers with their Orders, or in your case: Persons with their uploaded Photos, consider to use Enumerable.GroupJoin .

The other way round: if you have an item and you want the one and only item that the foreign key refers to: the Student with the School that he attends, the Order with the Customer who made the order, or in your case: the Person with the City that he lives in, consider to use Enumerable.Join

In a one-to-many, if you want the one item with the many sub-items start at the one-side and use GroupJoin. If you want the sub-item with the one item that the foreign key refers to, start at the many-side and use Join.

You want the Person with the many photos and the one address that the foreign key refers to.

var person = dbContext.Persons.GroupJoin(dbContext.UploadedPhotos,

person => person.PersonId,        // from every Person take the primary key
photo => photo.PersonId,          // from ever Photo take the foreign key to the Person

// parameter resultSelector: from every Person, with its zero or more Photos, make one new:
(person, photosOfThisPerson) => new
{
    PersonId = person.PersonId,
    Name = person.Name,
    NumberOfUploadedPhotos = photosOfThisPerson.Count(),

    Address = dbContext.Addresses.Where(address => address.AddressId == person.AddressId)
        FirstOrDefault(),
};