entity framework - EF code first add model related exist childs -

i try add new model db model have child exists , got error:

"a referential integrity constraint violation occurred: property value(s) of 'category.id' on 1 end of relationship not match property value(s) of 'lesson.categoryid' on other end."

for example:

public class category {     public int id { get; set; }     public string name{ get; set; }     public list<lesson> lessons{ get; set; } }  public class lesson {     public int id { get; set; }     public string name{ get; set; }     public list<category> categories { get; set; } } 

i think category id still 0 until db generate new id , lesson.categoryid filled in 0 not exist in categories

save code:

db.categories.add(obj); db.changetracker.entries()        .where(x => x.entity basemodel && x.state == entitystate.added && ((basemodel)x.entity).id > 0)                     .tolist().foreach(x => x.state = entitystate.unchanged); 

any idea how fix that? anyway