如何在我的LINQ查询中包含第二个表中的列

Question

I have a table called Ad (advertisement):

public class Ad
{
    public long AdId { get; set; }
    public long UserId { get; set; } // <-- this is advertiser not currentUser
    public string Description { get; set; }
}

Users can add zero or more advertisements to their favourites list, so I have created another table called Favourite :

public class Favourite
{
    public long FavouriteId { get; set; }
    public long AdId { get; set; }
    public long UserId { get; set; }
}

And I have a table called User :

public class User
{
    public long UserId { get; set; }
    public string Name { get; set; }
}

This is how I retrieve an ad, given currentUserId and adId :

public TEntity GetAd(long currentUser, long adId)
{
    return Context.Ad
        .Where(r.AdId == adId)
        // include a flag indicating the pair (adId and currentUserId) exist in Favouries table
        .FirstOrDefault();
}

What I want to include in the result is a flag, indicating if the ad is added to the favourites list or not?

Answer 1

In Entity Framework we have navigation properties. You can define navigation property on child table for the parent table row. For you case you can change your child entity like following:

 
 
 
  
  public class Favourite { public long FavouriteId { get; set; } public long AdId { get; set; } public long UserId { get; set; } public virtual Ad Ad { get; set; } }
 
  

and now the Entity Framework should take care of populating it for you and you can access the Ad related row of Favourite like :

return Context.Ad
        .Where(r.AdId == adId)
        Select(x => new 
                   { 
                      ad = x, 
                      IsFavourite = Context.Favourite.Any(y=> y.AdId = adId 
                                                    && y.UserId = currentUserid))
        .FirstOrDefault();

But in your case, You can write a query to know if the ad is favourite or not:

public UserFavouriteAd
{
    public Ad Ad { get; set; }
    public IsFavourite { get; set; }
}

you can create a Domain View Model and map it to that :

return Context.Ad
            .Where(r.AdId == adId)
            .ToList()
            Select(x => new UserFavouriteAd 
                       { 
                          ad = x, 
                          IsFavourite = Context.Favourite.Any(y=> y.AdId = adId 
                                                        && y.UserId = currentUserid))
            .FirstOrDefault();

and populate it:

 return Context.Ad .Where(r.AdId == adId) .ToList() Select(x => new UserFavouriteAd { ad = x, IsFavourite = Context.Favourite.Any(y=> y.AdId = adId && y.UserId = currentUserid)) .FirstOrDefault(); 

Answer 2

public object GetAd(long currentUser, long adId)
{
    return from a in Context.Favourite
           where a.UserId  == currentUser
           select new {
                      flag = a.AdId != null
                      a.FavouriteId,
                      //etc
                      };
}