Code First - Fluent API

Remarks#

There are two general ways of specifying HOW Entity Framework will map POCO classes to database tables, columns, etc.: Data Annotations and Fluent API.

While Data Annotations are a simple to read and understand, they lack of certain features such as specifying the “Cascade on Delete” behavior for an entity. The Fluent API on the other hand is a bit more complex to use, but provides a far more advanced set of features.

Mapping models

Primary Key

By using the .HasKey() method, a property can be explicitly configured as primary key of the entity.

using System.Data.Entity;    
// ..

public class PersonContext : DbContext
{
    // ..

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        // ..

        modelBuilder.Entity<Person>().HasKey(p => p.PersonKey);
    }
}

Composite Primary Key

By using the .HasKey() method, a set of properties can be explicitly configured as the composite primary key of the entity.

using System.Data.Entity;    
// ..

public class PersonContext : DbContext
{
    // ..

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        // ..

        modelBuilder.Entity<Person>().HasKey(p => new { p.FirstName, p.LastName });
    }
}

Maximum Length

By using the .HasMaxLength() method, the maximum character count can be configured for a property.

using System.Data.Entity;    
// ..

public class PersonContext : DbContext
{
    // ..

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        // ..

        modelBuilder.Entity<Person>()
                    .Property(t => t.Name)
                    .HasMaxLength(100);
    }
}

The resulting column with the specified column length:

Required properties (NOT NULL)

By using the .IsRequired() method, properties can be specified as mandatory, which means that the column will have a NOT NULL constraint.

using System.Data.Entity;    
// ..

public class PersonContext : DbContext
{
    // ..

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        // ..

        modelBuilder.Entity<Person>()
                    .Property(t => t.Name)
                    .IsRequired();
    }
}

The resulting column with the NOT NULL constraint:

Explict Foreign Key naming

When a navigation property exist on a model, Entity Framework will automatically create a Foreign Key column. If a specific Foreign Key name is desired but is not contained as a property in the model, it can be set explicitly using the Fluent API. By utilizing the Map method while establishing the Foreign Key relationship, any unique name can be used for Foreign Keys.

public class Company
{
    public int Id { get; set; }
}

public class Employee
{
    property int Id { get; set; }
    property Company Employer { get; set; }
}

public class EmployeeContext : DbContext
{
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Employee>()
                    .HasRequired(x => x.Employer)
                    .WithRequiredDependent()
                    .Map(m => m.MapKey("EmployerId"));
    }
}

After specifying the relationship, the Map method allows the Foreign Key name to be explicitly set by executing MapKey. In this example, what would have resulted in a column name of Employer_Id is now EmployerId.