如何使用枚举类型的显示名称对 nHibernate 中的枚举列进行排序

Question

I have a table Email which has a column EmailType . EmailType is a enumeration in nHibernate and stored as int in DB. I have extension method also to get name for the email type:

namespace Email
{
    public enum EmailType : int
    {
        PasswordReset = 0,
        EmailVerification = 1,
        AccountCreation = 2
    };

    public static class EmailType Extension
    {
      public static string Name(this EmailType self)
      {
        switch (self)
        {
          case EmailType.PasswordReset :
            return "Password Reset";
          case EmailType.EmailVerification :
            return "Email Verification";
          case EmailType.AccountCreation :
            return "Account Creation";
          default:
            return null;
        }
      }
    }
}

Then I can get name of the enum by using EmailType.PasswordReset.Name()

I am fetching list of Emails by creating nHibernate criteria:

var criteria = session.CreateCriteria<Email>();
criteria.SetFirstResult(startIndex).SetMaxResults(maxResults);
criteria.AddOrder(Order.Asc("EmailType"));
return criteria.List<Email>();

Problem with this is that it sorts rows with EmailType int stored in DB. Is there any way I can use EmailType.PasswordReset.Name() extension method of EmailType enum to sort with the Name of enum?

Answer 1

As your enums (in this case) in alphabetical order would be represented by 2, 1, 0, you could order the EmailType in reverse order and this would give you what you want.

criteria.AddOrder(Order.desc("EmailType"));

However, as you say, "this is a coincidence".

The reason it can't sort on the enum names is that it doesn't know what they are, so you need to find a way to tell it. There are two:

One way is to add a calculated field which converts the enum values into the names. I have done this with other databases and I belive @Formula is what you use in Hibernate.

The other way is to create another table in your database which contains the enum numbers and enum values. Then link the two tables in the query so that the enum value is returned.

In either case you apply the sort on the new field.

Answer 2

I faced the same issue and I found a way using projections

QueryOver<TRoot, TSubType>.OrderBy(IProjection projection)

with

queryOver
    .OrderBy(GetOrderByEnumProjection<TSubType, MyEnum>(x => x.EnumProperty, GetAllMyEnumValuesSortedByDisplayName(CultureInfo.CurrentCulture)));

private static IProjection GetOrderByEnumProjection<TObject, TEnum>(Expression<Func<TObject, object>> enumProperty, IReadOnlyList<TEnum> enumValues)
    where TEnum : Enum
{
    int index = enumValues.Count - 1;

    TEnum enumValue = enumValues[index];

    IProjection elseProjection = GetConditional(enumProperty, enumValue, index, Projections.Constant(-1));

    for (index = enumValues.Count - 2; index >= 0; index--)
    {
        enumValue = enumValues[index];

        elseProjection = GetConditional(enumProperty, enumValue, index, elseProjection);
    }

    return elseProjection;
}

private static IProjection GetConditional<TObject, TEnum>(Expression<Func<TObject, object>> enumProperty, TEnum value, int sortIndex, IProjection elseProjection)
    where TEnum : Enum
{
    return Projections.Conditional(
        Restrictions.Eq(Projections.Property<TObject>(enumProperty), value),
        Projections.Constant(sortIndex),
        elseProjection);
}

(Here you can find more details on how the resulting SQL looks like: Adding more than one condition in Projection.Conditionals for queryover .)