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

user_807851 0
Viewed 26418

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?

entity-framework-4.1 c#
6 Answers

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);
user_3734820
Related