实体框架更新一个父实体更新另一个父实体

Question

I'm new to entity framework. I have an SupplyItem entity

public class SupplyItem 
{
    public virtual int Id { get; set; }

    public virtual BaseProduct Product
    {
        get { return product; }
        set { product = value; }
    }

    public virtual Boolean IsPublic
    {
        get { return isPublic; }
        set { isPublic = value; }
    }
 }

When I add an supplyitem object for the first time, it is adding a product and IsPublic attribute correctly. Like this, I added two objects for supplyitem entity, both objects are referencing to same product.

Now, I changed isPublic attribute for second object of supplyitem entity and updates the entity like this

     UnitOfWork.Context.Entry(supplyItem1).State = EntityState.Modified;

Above code line, updates isPublic attribute correctly but it makes null Product for another object of supplyitem entity which is referencing to the same product.

I don't understand this behavior. Any pointer will be really helpful ! Thanks.

Answer 1

uh,when you update,you're not read from db,you new a SupplyItem and set id,set IsPublic,and save,so the object you new it,Product property is default (null)
when you do these,the whole object (every property) updated. so if you want update parts of fields,you should do like this

var entry = db.Entry(entity);
if (entry.State == EntityState.Detached)
{
    db.Set<TEntity>().Attach(entity);
}
entry.Property("propertyName").IsModified = true

the propertyName is a string ,it must be same as the property's Name you can do this with a lamda

Update<TEntity>(TEntity entity, params Expression<Func<TEntity, object>>[] updateProperty)

so in the function,you need find the propetyname from expression

public static string GetExpressionText<TModel, TProperty>(Expression<Func<TModel, TProperty>> ex)
    {
        if (ex.Body.NodeType == ExpressionType.MemberAccess)
        {
            var memExp = ex.Body as MemberExpression;
            if (memExp != null)
            {
                return memExp.Member.Name;
            }
        }
        else if (ex.Body.NodeType == ExpressionType.Convert)
        {
            var exp = ex.Body as UnaryExpression;
            return GetExpressionText(exp);
        }
        return string.Empty;
    }

    public static string GetExpressionText(UnaryExpression exp)
    {
        if (exp != null)
        {
            if (exp.Operand.NodeType == ExpressionType.MemberAccess)
            {
                var memExp = exp.Operand as MemberExpression;
                if (memExp != null)
                {
                    return memExp.Member.Name;
                }
            }
        }
        return string.Empty;
    }

hope these code can help you