I am receiving the following error when attempting to create the database:
One or more validation errors were detected during model generation:
Interaction_CauseElement_Source: : Multiplicity is not valid in Role 'Interaction_CauseElement_Source' in relationship 'Interaction_CauseElement'. Because the Dependent Role properties are not the key properties, the upper bound of the multiplicity of the Dependent Role must be '*'.
Interaction_EffectElement_Source: : Multiplicity is not valid in Role 'Interaction_EffectElement_Source' in relationship 'Interaction_EffectElement'. Because the Dependent Role properties are not the key properties, the upper bound of the multiplicity of the Dependent Role must be '*'.
I've seen this error in other Stack Overflow posts, but in the examples I found, the OP was trying for a 1-to-1 relationship in both directions between the tables. That is not what I am looking for.
Here is my model:
public class Element
{
[Key]
public int ID { get; set; }
[Required, MaxLength(64)]
public string Name { get; set; }
[MaxLength(200)]
public string Description { get; set; }
}
public class Interaction
{
[Key]
public int ID { get; set; }
[Index, Required]
public int CauseID { get; set; }
[Index, Required]
public int EffectID { get; set; }
[MaxLength(64)]
public string Location { get; set; }
[ForeignKey("CauseID")]
public virtual Element CauseElement { get; set; }
[ForeignKey("EffectID")]
public virtual Element EffectElement { get; set; }
}
Items in the Elements table are unique. A pair of elements can interact with each other in any number of locations. The CauseID/EffectID pair is not going to be unique.
The only other place I am changing the model is in the OnModelCreating method. I had received this error:
Introducing FOREIGN KEY constraint 'FK_dbo.Interactions_dbo.Elements_Cause' on table 'Interactions' may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints. Could not create constraint. See previous errors.
And had to create a cascade policy for the model. This code fixed that error:
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
//Prevent cyclic cascade on elements table
modelBuilder.Entity<Interaction>()
.HasRequired(i => i.CauseElement)
.WithRequiredDependent()
.WillCascadeOnDelete(false);
modelBuilder.Entity<Interaction>()
.HasRequired(i => i.EffectElement)
.WithRequiredDependent()
.WillCascadeOnDelete(false);
base.OnModelCreating(modelBuilder);
}
But then I received the cryptic "Multiplicity" error. It seems like it wants me to make public virtual Element CauseElement into a collection like public virtual ICollection<Element> CauseElement, but that would not properly model the relationship.
I found the solution. This article on EntityFrameworkTutoral.net helped out. Because I need TWO references from the Interaction class to the Element class, this relationship is too complex to model in EF with only the attributes.
I had to update the model and then use the fluent API to tell EF how to treat the relationships. I updated my model to the following:
public class Element
{
public Element()
{
CauseElements = new List<Interaction>();
EffectElements = new List<Interaction>();
}
[Key]
public int ID { get; set; }
[Required, MaxLength(64)]
public string Name { get; set; }
#region Navigation
public virtual ICollection<Interaction> CauseElements { get; set; }
public virtual ICollection<Interaction> EffectElements { get; set; }
#endregion
}
public class Interaction
{
[Key]
public int ID { get; set; }
[Index]
public int CauseID { get; set; }
[Index]
public int EffectID { get; set; }
[MaxLength(64)]
public string Location { get; set; }
#region Navigation
[ForeignKey("CauseID")]
public virtual Element CauseElement { get; set; }
[ForeignKey("EffectID")]
public virtual Element EffectElement { get; set; }
#endregion
}
And in my DbContext class I used the fluent API to create the link between the Interaction.CauseElement and Element.CauseElements and which property was the foreign key for the Interaction table (and the same with the Effect relationship):
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
//Prevent cyclic cascade on elements table
modelBuilder.Entity<Interaction>()
.HasRequired(i => i.CauseElement)
.WithRequiredDependent()
.WillCascadeOnDelete(false);
modelBuilder.Entity<Interaction>()
.HasRequired(i => i.EffectElement)
.WithRequiredDependent()
.WillCascadeOnDelete(false);
//Create the links between the element, the key, and the collection
modelBuilder.Entity<Interaction>()
.HasRequired<Element>(i => i.CauseElement)
.WithMany(e => e.CauseElements)
.HasForeignKey(i => i.CauseID);
modelBuilder.Entity<Interaction>()
.HasRequired<Element>(i => i.EffectElement)
.WithMany(e => e.EffectElements)
.HasForeignKey(i => i.EffectID);
base.OnModelCreating(modelBuilder);
}
It seems that Entity Framework tries to automatically infer the relationships between the tables when you have a simple 1-to-many relationship. If I removed EffectElement from the Interaction class (and EffectElements from Element), EF was able to create the relationship easily. But when I added it back, I received the error again.
Since that Element type showed up twice in the Interaction class, it didn't know how to create the relationship. I had to explicitly define it in the OnModelCreating method.
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