Getting class field names and table column names from NHibernate metadata

Question

Background

I am using a legacy database with all kinds of ugly corners. One bit is auditing. There is a table listing tablename/field combinations of fields that should have an audit trail. For example, if there is a row that has "WORKORDER" for the table name and "STATUS" for the fieldname, then I need to add row(s) to the auditing table whenever the Workorder.Status property changes in the application. I know the approach: NH events or interceptors, but I've got an issue to figure out before I get to that stage.

Question

What I need to know is how to get a list of key/value pairs for a single persistent class containing (a) the database field name and (b) the associated property name in the class. So for my example, I have a class called Workorder associated with a table called (no surprise) WORKORDER. I have a property on that Workorder class called CurrentStatus. The matching property in the WORKORDER table is STATUS. Notice the mismatch between the property name and table column name? I need to know the property name to access the before and after data for the audit. But I also need to know the backing column name so that I can query the stupid legacy "AuditTheseColumns" table.

What I've tried

in my application I change the Workorder.CurrentStatus from "TS" to "IP". I look in my audit tracking table and see that the WORKORDER.STATUS column is tracked. So after calling Session.SaveOrUpdate(workorder), I need to find the Workorder property associated with the STATUS column and do a Session.Save(auditRecord) telling it the old ("TS") and new ("IP") values.

As far as I can tell, you can get information about the class:

        var fieldNames = new List<string>();
        IClassMetadata classMetadata = SessionFactory(Resources.CityworksDatasource).GetClassMetadata(typeof(T));
        int propertyCount = 0;
        foreach (IType propertyType in classMetadata.PropertyTypes)
        {
            if (propertyType.IsComponentType)
            {
                var cp = (ComponentType)propertyType;

                foreach (string propertyName in cp.PropertyNames)
                {
                    fieldNames.Add(propertyName);
                }
            }
            else if(!propertyType.IsCollectionType)
            {
                fieldNames.Add(classMetadata.PropertyNames[propertyCount + 1]);
            }

            propertyCount++;
        }

And information about the table:

        var columnNames = new List<string>();
        PersistentClass mappingMeta = ConfigureCityworks().GetClassMapping(typeof(T));

        foreach (Property property in mappingMeta.PropertyIterator)
        {
            foreach (Column selectable in property.ColumnIterator)
            {
                if (columnNames.Contains(selectable.Name)) continue;
                columnNames.Add(selectable.Name);
            }
        }

But not at the same time. Any ideas? I'm at a loss where to look next.

Answer 1

How to get the database column / field names and class property names for an entity mapped by NHibernate:

using System;
using System.Collections.Generic;
using System.Reflection;
using NHibernate;
using NHibernate.Persister.Entity;

namespace Stackoverflow.Example
{
    /// <summary>
    /// NHibernate helper class
    /// </summary>
    /// <remarks>
    /// Assumes you are using NHibernate version 3.1.0.4000 or greater (Not tested on previous versions)
    /// </remarks>
    public class NHibernateHelper
    {
        /// <summary>
        /// Creates a dictionary of property and database column/field name given an
        /// NHibernate mapped entity
        /// </summary>
        /// <remarks>
        /// This method uses reflection to obtain an NHibernate internal private dictionary.
        /// This is the easiest method I know that will also work with entitys that have mapped components.
        /// </remarks>
        /// <param name="sessionFactory">NHibernate SessionFactory</param>
        /// <param name="entity">An mapped entity</param>
        /// <returns>Entity Property/Database column dictionary</returns>
        public static Dictionary<string, string> GetPropertyAndColumnNames(ISessionFactory sessionFactory, object entity)
        {
            // Get the objects type
            Type type = entity.GetType();

            // Get the entity's NHibernate metadata
            var metaData = sessionFactory.GetClassMetadata(type.ToString());

            // Gets the entity's persister
            var persister = (AbstractEntityPersister)metaData;

            // Creating our own Dictionary<Entity property name, Database column/filed name>()
            var d = new Dictionary<string, string>();

            // Get the entity's identifier
            string entityIdentifier = metaData.IdentifierPropertyName;

            // Get the database identifier
            // Note: We are only getting the first key column.
            // Adjust this code to your needs if you are using composite keys!
            string databaseIdentifier = persister.KeyColumnNames[0];

            // Adding the identifier as the first entry
            d.Add(entityIdentifier, databaseIdentifier);

            // Using reflection to get a private field on the AbstractEntityPersister class
            var fieldInfo = typeof(AbstractEntityPersister)
                .GetField("subclassPropertyColumnNames", BindingFlags.NonPublic | BindingFlags.Instance);

            // This internal NHibernate dictionary contains the entity property name as a key and
            // database column/field name as the value
            var pairs = (Dictionary<string, string[]>)fieldInfo.GetValue(persister);

            foreach (var pair in pairs)
            {
                if (pair.Value.Length > 0)
                {
                    // The database identifier typically appears more than once in the NHibernate dictionary
                    // so we are just filtering it out since we have already added it to our own dictionary
                    if (pair.Value[0] == databaseIdentifier)
                        break;

                    d.Add(pair.Key, pair.Value[0]);
                }
            }

            return d;
        }
    }
}

Usage:

// Get your NHiberate SessionFactory wherever that is in your application
var sessionFactory = NHibernateHelper.SessionFactory;

// Get an entity that you know is mapped by NHibernate
var customer = new Customer();

// Get a dictionary of the database column / field names and their corresponding entity property names
var propertyAndColumnNamesDictionary =
    Stackoverflow.Example.NHibernateHelper.GetPropertyAndColumnNames(sessionFactory, customer);