Posts Tagged Int Id

Data Annotations and Entity Framework Code First

Posted by John in .Net on November 5, 2010

I like the code first model of Entity Framework 4 (CTP2 at time of writing). A Domain Entity can be decorated with data annotations which can then be used by the Entity Framework when constructing the database.

I have no concerns adding the System.ComponentModel.DataAnnotations reference to my Domain; I have no intention of changing my DAL and if I change my database it will only be to a different flavour of MS SQL.

There are a number of ‘purposes’ for data annotations but they don’t all sit comfortable within a domain model.

They can represent business rules: 

[Key]
public string AccountNumber { get; set; }
[Required]
[StringLength(50)]
public string Name { get; set; }

The KeyAttribute really could be considered a database hint or a business rule.

They can represent hints to the database:

[StoreGenerated(StoreGeneratedPattern.Identity)]
public int AccountNumber { get; set; }

They can also be hints to the UI:

[ScaffoldColumn(false)]
public int Id { get; set; }

Whilst I like the idea of keeping all these annotations neatly tucked up inside my model, the purist in me wants to move the UI hints to a view model and the database hints to an entity configuration class in the data layer.

I think I’ll stick with the KISS approach and keep the annotations within my domain model but reserve the right to change my mind. Any comments / views for or against are welcome.

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Entity Classes, IDs and Equality

Posted by John in Methodology on April 22, 2010

I want my entity classes to rely on their ID when checking equality, the ID is populated from the repository when I retrieve an object and automatically generated when I save a new entity.

This poses the problem of how I compare two entities before they have been saved while working with them in my domain, or, when I have ‘newed’ them up myself for unit testing. The answer is to use a transient ID.

Normally an Entity Base class might look like this:

public abstract class EntityBase : IEquatable<EntityBase> {

    public virtual int Id { get; protected set; }

    public override bool Equals(object other) {
        return Equals(other as EntityBase);
    }

    public override int GetHashCode() {
        return Id;
    }

    public virtual bool Equals(EntityBase other) {
        if (other == null) return false;
        return other.Id == Id && other.GetType() == GetType();
    }

}

If two new objects are created from a derived class then object1.Equals(object2) will be true as Id will be 0 for both objects. To avoid this I need to check if an object has not yet been saved:

private bool IsTransient {
    get { return Id == 0; }
}

And just check for referential equality if this is the case:

public virtual bool Equals(EntityBase other) {
    if (other == null) return false;
    if (IsTransient) return ReferenceEquals(this, other);
    return other.Id == Id && other.GetType() == GetType();
}

GetHashCode() needs to be overridden to reflect this:

public override int GetHashCode() {
    if (IsTransient) return base.GetHashCode();
    return Id;
}

The full code for EntityBase is:

public abstract class EntityBase : IEquatable<EntityBase> {

    public virtual int Id { get; protected set; }

    private bool IsTransient {
        get { return Id == 0; }
    }

    public override bool Equals(object other) {
        return Equals(other as EntityBase);
    }

    public override int GetHashCode() {
        if (IsTransient) return base.GetHashCode();
        return Id;
    }

    public virtual bool Equals(EntityBase other) {
        if (other == null) return false;
        if (IsTransient) return ReferenceEquals(this, other);
        return other.Id == Id && other.GetType() == GetType();
    }

}

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