Custom Code-First Conventions:

Code-First has a set of default behaviors for the models that are referred to as conventions. EF 6 provides the ability to define your own custom conventions which will be the default behavior for your models.

There are two main types of Conventions, Configuration Conventions and Model Conventions.

Configuration Conventions:

Configuration Conventions are a way to configure entities without overriding the default behavior provided in the Fluent API. You can define a configuration convention inside your OnModelCreating event or in a custom Convention Class, similar to how you would define normal entity mappings with the Fluent API.

For example, you can define a primary key of an entity that has property named {entity name} _ID, e.g. Student_ID property of Student entity will be primary key:

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    modelBuilder
        .Properties()
        .Where(p => p.Name == p.DeclaringType.Name + "_ID")
        .Configure(p => p.IsKey());

    base.OnModelCreating(modelBuilder);
}

The following example shows how to set the string length of the Description property in all entities:

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
        modelBuilder
            .Properties()
            .Where(p => p.Name == "Description")
            .Configure(p => p.HasMaxLength(200));

    base.OnModelCreating(modelBuilder);
}

You can also define custom class for this convention by deriving the Convention class as shown below:

public class PKConvention : Convention
{
    public PKConvention()
    {
        .Properties()
        .Where(p => p.Name == p.DeclaringType.Name + "_ID")
        .Configure(p => p.IsKey());

    }
}

Configure custom convention as shown below:

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    modelBuilder.Conventions.Add<PKConvention>();
}

Model Conventions:

Model Conventions are based on the underlying model metadata. There are conventions for both CSDL and SSDL. Create a class that implements IConceptualModelConvention from CSDL conventions and implement IStoreModelConvention for SSDL convention.

Visit codeplex documentation for more information.

Download code-first sample project for custom conventions demo.