I have two classes. How can I turn these two classes into a one to one relationship using the entity framework code first method?
public class Region { public int RegionId { get; set; } public string Name { get; set; } public virtual Factory _factory { get; set; } } public class Factory { public int FactoryId { get; set; } public string Name { get; set; } public virtual Region _region { get; set; } }
When I try this, I get this error: Multiplicity is not valid in Role 'Region_Factory_Source' in relationship 'Region_Factory'. Because the Dependent Role properties are not the key properties, the upper bound of the multiplicity of the Dependent Role must be '*'.
Step 1 − First, create the console application from File → New → Project… Step 2 − Select Windows from the left pane and Console Application from the template pane. Step 3 − Enter EFCodeFirstDemo as the name and select OK. Step 4 − Right-click on your project in the solution explorer and select Manage NuGet Packages…
A one-to-many relationship happens when the primary key of one table becomes foreign keys in another table. The foreign key is defined in the table that represents the many end of the relationship.
“HasMany” and “WithMany” method is used to define one-to-many or many-to-many relation in entity framework. We can configure one-to-many relationships between People and PeopleAddress using Fluent API by the following code in the model class: protected override void OnModelCreating(DbModelBuildermodelBuilder) {
One way to implement a one-to-one relationship in a database is to use the same primary key in both tables. Rows with the same value in the primary key are related. In this example, France is a country with the id 1 and its capital city is in the table capital under id 1.
This occurs in CodeFirst because of the virtual keyword. In effect, you are creating a relationship where creating one item requires the creation of the other. however, the virtual keyword allows lazy instantiation, which means that creating an object of one type doesn't automatically create the other type, allowing the Id on the foreign item to be null. This implies a 0..1 relationship, but since each side is virtual, what you get is a 0..0 which isn't allowed.
There are 2 methods which you can use to remedy the situation.
Region
add a property for FactoryId
and on Factory
add a property for RegionId
There are other ways to help Entity Framework determine which object is the Dependent Object, i.e. using Entity Framework Fluent api.
from MSDN
Configuring a Relationship Where Both Ends Are Required (One-to-One)
In most cases the Entity Framework can infer which type is the dependent and which is the principal in a relationship. However, when both ends of the relationship are required or both sides are optional the Entity Framework cannot identify the dependent and principal. When both ends of the relationship are required, use WithRequiredPrincipal or WithRequiredDependent after the HasRequired method. When both ends of the relationship are optional, use WithOptionalPrincipal or WithOptionalDependent after the HasOptional method.
the following code would create a Principal Factory
with a Dependent Region
// Configure the primary key for the Region modelBuilder.Entity<Region>() .HasKey(t => t.RegionId); modelBuilder.Entity<Factory>() .HasRequired(t => t.Region) .WithRequiredPrincipal(t => t.Factory);
EF6, add attributes:
[Key] public int RegionId { get; set; } [Key, ForeignKey("Region")] public int FactoryId { get; set; }
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