ef code first - Entity Framework - 3 tables have relationships with each other -

i have 3 tables follows:

applicationuser:

public class applicationuser : identityuser {     ..some basic properties..      // navigation properties     public virtual icollection<post> posts { get; set; }     public virtual icollection<album> albums { get; set; } } 

post:

public class post {         public long id { get; set; }             public string content { get; set; }          public int? albumid { get; set; }         public string userid { get; set; }          public virtual applicationuser user { get; set; }         public virtual album album { get; set; } } 

album:

public class album {         public int id { get; set; }         public string name { get; set; }         public string userid { get; set; }          public virtual applicationuser user { get; set; }         public virtual icollection<post> posts { get; set; } } 

and applicationdbcontext:

modelbuilder.entity<applicationuser>()                 .hasmany(a=>a.posts)                 .withrequired(a=>a.user)                 .hasforeignkey(a=>a.userid)                 .willcascadeondelete(false); modelbuilder.entity<post>()                 .haskey(p => p.id);  modelbuilder.entity<album>()                 .haskey(a => a.id);  modelbuilder.entity<applicationuser>()                 .hasmany(u=>u.albums)                 .withoptional()                 .hasforeignkey(a=>a.userid)                 .willcascadeondelete();  modelbuilder.entity<album>()                 .hasmany(a=>a.posts)                 .withrequired()                 .hasforeignkey(p=>p.albumid)                 .willcascadeondelete(); 

when run migration , update database, error:

the alter table statement conflicted foreign key constraint "fk_dbo.posts_dbo.albums_albumid". conflict occurred in database "aspnet-link-20161012104217", table "dbo.albums", column 'id'.

could tell why conflict? seems pretty legit me.

in code set albumid nullable in configuration defined withrequeired():

public class post {     public long id { get; set; }         public string content { get; set; }      public int? albumid { get; set; }       //<-- 1     public string userid { get; set; }      public virtual applicationuser user { get; set; }     public virtual album album { get; set; } }  modelbuilder.entity<album>()             .hasmany(a=>a.posts)             .withrequired() //<-- 1             .hasforeignkey(p=>p.albumid)             .willcascadeondelete(); 

if albumid nullable should change configuration:

//ef default conventions set albumid foreign key modelbuilder.entity<album>()             .hasmany(a=>a.posts)             .withoptional(a=>a.album); 

and if albumid isn't nullable change property:

public class post {     public long id { get; set; }         public string content { get; set; }      public int albumid { get; set; }          public string userid { get; set; }      public virtual applicationuser user { get; set; }     public virtual album album { get; set; } } 

and use following configuration:

//ef default conventions set albumid foreign key modelbuilder.entity<album>()             .hasmany(a=>a.posts)             .withrequired(a=>a.album)             .willcascadeondelete();