Equality between custom objects in Marten Linq

[xevilmaxx]

Hi,
I have folloving structure:

public class DevConfig
    {
        public string Key { get; set; }
        public dynamic Value { get; set; }
        public string Description { get; set; }
    }

public class JsonDeviceConfigs
    {
        public long Id { get; set; }
        public int DeviceType { get; set; }
        public string Name { get; set; }
        public string Description { get; set; }
        public List<DevConfig> Configs { get; set; }
    }

When i perform following query, i receive error that Marten currently not handling System.Object.Equals() method

var newcfg = new DevConfig() { Key = "listening_port", Value = 50051, Description = "Own Server Port Number [0 => let choose to GRPC]" };

var res4 = session.Query<JsonDeviceConfigs>()
                    .SelectMany(x => x.Configs, (parent, child) => child)
                    .Where(x => x.Equals(newcfg))
                    .ToList();

//alternatively can be also written like that:
var res2 = (
 from dc in session.Query<JsonDeviceConfigs>()
 from cfgs in dc.Configs
 where cfgs.Equals(newcfg)
 select cfgs
);

---

So here is my temporal workaround:
I started from this:
https://martendb.io/documentation/documents/advanced/customizing_linq/

Context = DocumentStore.For(_ => 
                    {

                        //Custom connection string
                        _.Connection(connectionString);

                        _.NameDataLength = 250;

                        //Custom Schema Name
                        _.DatabaseSchemaName = "ParkO_RTS";
                        //My custom override for comparing objects not supported by Marten natively (for now alpha-8)
                        _.Linq.MethodCallParsers.Add(new LinqEqualsExt());

                    });

Than i have created a satatic extension es explained in guide and generalized:

public static class CustomExtensions
    {
        /// <summary>
        /// Overriden by my custom Linq for Marten
        /// </summary>
        /// <param name="src"></param>
        /// <param name="dst"></param>
        /// <returns></returns>
        public static bool MyEquals(this object src, object dst)
        {
            return true;// src.Equals(dst);
        }

    }

Also i have this extension that tries to find equality in objects:

public class LinqEqualsExt : IMethodCallParser
    {
        /// <summary>
        /// Hook to a static function
        /// </summary>
        /// <param name="expression"></param>
        /// <returns></returns>
        public bool Matches(MethodCallExpression expression)
        {
            return expression.Method.Name == nameof(CustomExtensions.MyEquals);
        }

        /// <summary>
        /// Mainly it overrides Original Method defined in CustomExtensions.MyEquals
        /// </summary>
        /// <param name="mapping"></param>
        /// <param name="serializer"></param>
        /// <param name="expression"></param>
        /// <returns></returns>
        public ISqlFragment Parse(IFieldMapping mapping, ISerializer serializer, MethodCallExpression expression)
        {
            //get object witch which we want to compare
            var compareTo = expression.Arguments[1].Value();
            
            //get all of its properties
            PropertyInfo[] _properties = compareTo.GetType().GetProperties();

            string addedSql = "";
            var lastProp = _properties.Last();

            for (int i = 0; i < _properties.Length; i++)
            {
                var locator = mapping.FieldFor(new MemberInfo[] { _properties[i] }).TypedLocator;
                if (i == (_properties.Length - 1))
                {
                    //last element of an object
                    addedSql += $"{locator} = '{_properties[i].GetValue(compareTo)}'";
                }
                else
                {
                    addedSql += $"{locator} = '{_properties[i].GetValue(compareTo)}' and ";
                }
            }

            return new WhereFragment(addedSql);
        }

    }

In the end it seems to work.

var res4 = session.Query<JsonDeviceConfigs>()
                    .SelectMany(x => x.Configs, (parent, child) => child)
                    .Where(x => x.MyEquals(newcfg))
                    .ToList();

Hope at least equality between objects feature will be added sooner or later.
Its not an issue but i wonder if there is a better way to perform this operation or its already ok like that.

Thanks in advance.

[jeremydmiller]

@xevilmaxx I think at most this is something that we could document, with your example as a workaround. You could do this a little more efficiently by using a custom IField and matching IFieldSource to cache the locators. It's not something that Marten could ever generalize because you're needing to compare the value against the serialized JSON in the database.