Setting a foreign key to null when using entity framework code first

Question

I'm using the database first implementation of Entity Framework Code First as the data layer for a project, but I've run into a problem.

I need to be able to set a foreign key to null in order to remove an association in the database.

I have 2 objects. One is called Project.

public class Project
{
    public int ProjectId {get; set;}
    public Employee Employee {get;set;}
}

public class Employee
{
    public int EmployeeId {get; set;}
    public string EmployeeName {get;set;}
}

This matches what I have in the Database:

CREATE TABLE Project(
    ProjectId int IDENTITY(1,1) NOT NULL,
    EmployeeId int NULL
)

CREATE TABLE Project(
    EmployeeId int IDENTITY(1,1) NOT NULL,
    EmployeeName varchar(100) NULL
)

I can assign an Employee to a project. However, I want to be able to remove an employee from a project and have the Employee field be null. In my UI this will show as 'No EMployee Assigned'.

However, short of a direct sql query, I cannot seem to find a way to do this in the entity framework 4.1.

I've tried:

public void RemoveEmployeeFromProject(int projectId)
{
    var project = Context.Projects.FirstOrDefault(x => x.ProjectId == projectId);
    project.Employee = null;
    Context.SaveChanges();
}

But this doesn't do anything.

Does anyone have any ideas?

Answer 1

I think the problem is that as far as the context is concerned, you haven't actually changed anything.

You can use the lazy loading approach previously suggested by using virtual, but since you haven't requested that the Employee be loaded yet, it's still null. You could try this:

var forceLoad = project.Employee;
project.Employee = null; // Now EF knows something has changed
Context.SaveChanges();

Alternatively, explicitly include it in your original request:

var project = Context.Projects.Include(x => x.Employee).FirstOrDefault(x => x.ProjectId == projectId);
project.Employee = null;
Context.SaveChanges();

On a side note, FirstOrDefault will return null if no Project matches the given id. If you know the project exists, you can just use First. You could even use Single which will assert that there is only one such project. If you continue to use FirstOrDefault, I'd recommend checking for null before working with project.

Answer 2

You can do it this way, which means you don't have to load the related entity.

context.Entry(Project).Reference(r => r.Employee).CurrentValue = null;

Answer 3

The answer to this is quite simple. EF can't infer the type given the information you've provided.

Just do this instead:

public void RemoveEmployeeFromProject(int projectId)
{
    var project = Context.Projects.FirstOrDefault(x => x.ProjectId == projectId);
    project.EmployeeId = (int?)null;
    Context.SaveChanges();
}

and it will work.

Answer 4

You need to include in the linq query, the property to assign, using the same name it has in the Project class:

var project = Context.Projects.Include("Employee").FirstOrDefault(x => x.ProjectId == projectId);

Answer 5

As another workaround, I compiled two methods into a extension method:

public static void SetToNull<TEntity, TProperty>(this TEntity entity, Expression<Func<TEntity, TProperty>> navigationProperty, DbContext context = null)
    where TEntity : class
    where TProperty : class
{
    var pi = GetPropertyInfo(entity, navigationProperty);

    if (context != null)
    {
        //If DB Context is supplied, use Entry/Reference method to null out current value
        context.Entry(entity).Reference(navigationProperty).CurrentValue = null;
    }
    else
    {
        //If no DB Context, then lazy load first
        var prevValue = (TProperty)pi.GetValue(entity);
    }

    pi.SetValue(entity, null);
}

static PropertyInfo GetPropertyInfo<TSource, TProperty>(    TSource source,    Expression<Func<TSource, TProperty>> propertyLambda)
{
    Type type = typeof(TSource);

    MemberExpression member = propertyLambda.Body as MemberExpression;
    if (member == null)
        throw new ArgumentException(string.Format(
            "Expression '{0}' refers to a method, not a property.",
            propertyLambda.ToString()));

    PropertyInfo propInfo = member.Member as PropertyInfo;
    if (propInfo == null)
        throw new ArgumentException(string.Format(
            "Expression '{0}' refers to a field, not a property.",
            propertyLambda.ToString()));

    if (type != propInfo.ReflectedType &&
        !type.IsSubclassOf(propInfo.ReflectedType))
        throw new ArgumentException(string.Format(
            "Expression '{0}' refers to a property that is not from type {1}.",
            propertyLambda.ToString(),
            type));

    return propInfo;
}

This allows you to supply a DbContext if you have one, in which case it will use the most efficient method and set the CurrentValue of the Entry Reference to null.

entity.SetToNull(e => e.ReferenceProperty, dbContext);

If no DBContext is supplied, it will lazy load first.

entity.SetToNull(e => e.ReferenceProperty);

Answer 6

if you enable lazy loading by making the employee property virtual does it work?

public class Project
{
    public int ProjectId {get; set;}
    public virtual Employee Employee {get;set;}
}

i'd also suggest encapsulating the remove method as part of your poco class to making the meaning more clear. see this article for more details on that.

public class Project
{
    public int ProjectId {get; set;}
    public virtual Employee Employee {get;set;}
    public void RemoveEmployee()
    {
        Employee = null;
    }
}