Adding data to Tables with Navigation Properties

Question

User1439689341 posted

I have a created Entity Framework.I am trying to add a new songs and New album to the table. For example when I try to add a new song I used the method below and it tells me "Object reference not set to an instance of an object" on the s.Artist.ArtistName. I have a webform that shows a list of Artists in a drop down menu and now I want to add a new song for that artist. How do I do that?

public Song AddNewSongNav(String ArtistName, String SongName)
{
    using (var context = new MyEntities())
    {
        var s = new Song();
        s.SongTitle = SongName;
        s.Artist.ArtistName = ArtistName;
        s.Artist.WikipediaUrl="http://en.wikipedia.org/Testing";

        context.Songs.AddObject(s);
        context.SaveChanges();
        return s;
    }
}

The frame work for the Artist Table has ArtistID,ArtistName and WikiPediaURL. It has a navivgation property for Album and Song. It is linked to Song as 1 to many and Album as 1 to Many.

It has a Song table, which has a SongID, SongTitle and Artist_ArtistID. It has a navigation property of Artist and Album. It's linked as a many to many to the Albums table.

Finally the Album table has an AlbumID,AlbumTitle,CoverArt,Year,Genre,MimeType and Artist_ArtistID. It has a navigation property of Artist and Song.

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Answer 1

User-1877821760 posted

http://www.dotnetcurry.com/ShowArticle.aspx?ID=135

Answer 2

User3866881 posted

Hello:)

【Solutions】

Check it before doing inserting an object into s.Artist——I suspect whether Artist is null or not…

using (var context = new MyEntities())
    {
        var s = new Song();
        s.SongTitle = SongName;
        if(s.Artist==null)
        {
          s.Artist = new Artist();
        }
        s.Artist.ArtistName = ArtistName;
        s.Artist.WikipediaUrl="http://en.wikipedia.org/Testing";

        context.Songs.AddObject(s);
        context.SaveChanges();
        return s;
    }

【Reasons】

s.Artist may be null。

Answer 3

User-330204900 posted

if I understand you correctly you have an Edit or Insert page in DD (Dynmaic Data) and this has several drop down lists for selecting entries for the page but you need a methos where you can add a new entry to one or more of the drop dwon lists?

if this is correct you can use my A Popup Insert control for Dynamic Data which allows this in DD.