Move database/mapper setup code to Core

PluralKit.Core/DatabaseUtils.cs

@@ -0,0 +1,55 @@
+using System.Data;
+using Dapper;
+using NodaTime;
+using Npgsql;
+
+namespace PluralKit
+{
+    public static class DatabaseUtils
+    {
+        public static void Init()
+        {
+            // Dapper by default tries to pass ulongs to Npgsql, which rejects them since PostgreSQL technically
+            // doesn't support unsigned types on its own.
+            // Instead we add a custom mapper to encode them as signed integers instead, converting them back and forth.
+            SqlMapper.RemoveTypeMap(typeof(ulong));
+            SqlMapper.AddTypeHandler<ulong>(new UlongEncodeAsLongHandler());
+            Dapper.DefaultTypeMap.MatchNamesWithUnderscores = true;
+
+            // Also, use NodaTime. it's good.
+            NpgsqlConnection.GlobalTypeMapper.UseNodaTime();
+            // With the thing we add above, Npgsql already handles NodaTime integration
+            // This makes Dapper confused since it thinks it has to convert it anyway and doesn't understand the types
+            // So we add a custom type handler that literally just passes the type through to Npgsql
+            SqlMapper.AddTypeHandler(new PassthroughTypeHandler<Instant>());
+            SqlMapper.AddTypeHandler(new PassthroughTypeHandler<LocalDate>());
+        }
+        
+        class UlongEncodeAsLongHandler : SqlMapper.TypeHandler<ulong>
+        {
+            public override ulong Parse(object value)
+            {
+                // Cast to long to unbox, then to ulong (???)
+                return (ulong)(long)value;
+            }
+
+            public override void SetValue(IDbDataParameter parameter, ulong value)
+            {
+                parameter.Value = (long)value;
+            }
+        }
+
+        class PassthroughTypeHandler<T> : SqlMapper.TypeHandler<T>
+        {
+            public override void SetValue(IDbDataParameter parameter, T value)
+            {
+                parameter.Value = value;
+            }
+
+            public override T Parse(object value)
+            {
+                return (T) value;
+            }
+        }
+    }
+}