Why do I get a "relationship multiplicity violation" accessing a relation property? - entity-framework

Question

I have the following classes in which I am trying to map the entity object to the view model:
public class ConsumerIndexItem: MappedViewModel<Consumer>
{
public string UserName { get; set; }
public string RoleDescription { get; set; }
public override void MapFromEntity(Consumer entity)
{
base.MapFromEntity(entity);
UserName = entity.User.UserName;
}
}
public class Consumer: AuditableEntity
{
public virtual User User { get; set; }
public virtual Role Role { get; set; }
}
public class IndexModel<TIndexItem, TEntity> : ViewModel where TEntity : new()
{
public IndexModel()
{
Items = new List<TIndexItem>();
}
public List<TIndexItem> Items { get; set; }
public virtual void MapFromEntityList(IEnumerable<TEntity> entityList)
{
Items = Mapper.Map<IEnumerable<TEntity>, List<TIndexItem>>(entityList);
}
}
public class ConsumerIndexModel: IndexModel<ConsumerIndexItem, Consumer>
However, if I drive the mapping with the following code:
var model = new ConsumerIndexModel();
var list = _repository.List().Where(c => c.Parent == null).ToList();
model.MapFromEntityList(list);
return View(model);
on the line UserName = entity.User.UserName; in ConsumerIndexItem I get the following exception:
A relationship multiplicity constraint violation occurred: An EntityReference can have no more than one related object, but the query returned more than one related object. This is a non-recoverable error.
If I execute ?entity.User.UserName in the Immediate Window I get the expected user name value. What could be wrong here?

Answer 1

Let me explain why I had this exception and you may be able to correlate it with your own situation. I had EF Code First model mapped to the existing database. There was one-to-many relationship between two of the entities. The child table had composite primary consisting of the Id and Date. However, I missed the second segment of the primary key in my fluent map:
this.HasKey(t => t.Id);
The strange part of that was that the model worked but was throwing an exception in certain cases and it was very hard to understand why. Apparently when EF was loading the parent of child entity there were more than one parent since the key had not only Id but Date as well. The resolution was to include the second part of the key:
this.HasKey(t => new { t.Id, t.Date });
The tool that helped me to pinpoint the problem was EF Power Tools, currently it is in Beta 3. The tool gives a context menu for the EF context class where one the item is View Entity Model DDL SQL. Although I could have found this just by checking the code, the tool is nice in showing how close the EF model matches the actual database.
I believe that you're getting this exception because for a some reasons the multiplicity of relationship is violated. In my case it was incorrect mapping, in your it may be something else, I can't tell by looking at your code.

Answer 2

I think the problem maybe that you suppose that every user has only one consumer while this is not correct regarding data.
I had the same problem and it was because the relationship was on-to-many and I made it one-to-one.