JsonMappableType.cs

// Implementation of custom NHibernate type, that helps to store and retrive JSON format
// JsonFormatter - static class with Serialize/Deserialize methods (with JsonConvert implementation for example)

namespace Tm.NHibernate.Types
{
    public class JsonMappableType<T> : IUserType where T : class
    {
        public new bool Equals(object x, object y)
        {
            if (x == null && y == null)
                return true;

            if (x == null || y == null)
                return false;

            return JsonFormatter.Serialize(x) == JsonFormatter.Serialize(y);
        }

        public int GetHashCode(object x)
        {
            if (x == null)
                return 0;

            return x.GetHashCode();
        }

        public object NullSafeGet(IDataReader rs, string[] names, object owner)
        {
            if(names.Length != 1)
                throw new InvalidOperationException("Expect only one column");

            var val = rs[names[0]] as string;

            if (!string.IsNullOrWhiteSpace(val))
            {
                return JsonFormatter.Deserialize<T>(val);
            }

            return null;
        }

        public void NullSafeSet(IDbCommand cmd, object value, int index)
        {
            var parameter = (DbParameter) cmd.Parameters[index];

            parameter.Value = value == null 
                ? (object) DBNull.Value 
                : JsonFormatter.Serialize(value);
        }

        public object DeepCopy(object value)
        {
            if (value == null)
                return null;

            var serialized = JsonFormatter.Serialize(value);

            return JsonFormatter.Deserialize<T>(serialized);
        }

        public object Replace(object original, object target, object owner)
        {
            return original;
        }

        public object Assemble(object cached, object owner)
        {
            var str = cached as string;

            if (string.IsNullOrWhiteSpace(str))
                return null;

            return JsonFormatter.Deserialize<T>(str);
        }

        public object Disassemble(object value)
        {
            if (value == null)
                return null;

            return JsonFormatter.Serialize(value);
        }

        public SqlType[] SqlTypes 
            => new SqlType[] {new StringClobSqlType() };

        public Type ReturnedType 
            => typeof(T);

        public bool IsMutable 
            => true;
    }
}

Scenario.cs

// Usage example

public class Scenario {
    public virtual ISet<Call> Calls { get; set; }
}

public class ScenarioMap {
    public ScenarioMap() {
        Map(x => x.Calls).CustomType<JsonMappableType<ISet<Call>>>();
    }
}