Events-MVC (example with htmx)

Events-MVC/Events.MVC/Util/Middleware/ProblemDetailsForSqlException.cs

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+using Events.MVC.Util.Extensions;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.Filters;
+using Microsoft.EntityFrameworkCore;
+using Npgsql;
+
+namespace Events.MVC.Util.Middleware;
+
+public class ProblemDetailsForSqlException : ExceptionFilterAttribute
+{
+  private readonly ILogger<ProblemDetailsForSqlException> logger;
+
+  public ProblemDetailsForSqlException(ILogger<ProblemDetailsForSqlException> logger)
+  {
+    this.logger = logger;
+  }
+
+  public override void OnException(ExceptionContext context)
+  {
+    Exception? exception = context.Exception;
+    PostgresException? postgresException = null;
+
+    while (exception is not null)
+    {
+      if (exception is PostgresException currentPostgresException)
+      {
+        postgresException = currentPostgresException;
+        break;
+      }
+
+      if (exception is DbUpdateException dbUpdateException && dbUpdateException.InnerException is not null)
+      {
+        exception = dbUpdateException.InnerException;
+        continue;
+      }
+
+      exception = exception.InnerException;
+    }
+
+    if (postgresException is null)
+    {
+      base.OnException(context);
+      return;
+    }
+
+    ProblemDetails problemDetails = postgresException.SqlState switch
+    {
+      PostgresErrorCodes.UniqueViolation => new ProblemDetails
+      {
+        Title = "Duplicate data",
+        Detail = "A record with the same data already exists."
+      },
+      PostgresErrorCodes.ForeignKeyViolation => new ProblemDetails
+      {
+        Title = "Related data",
+        Detail = "The operation is not allowed because related data exists."
+      },
+      _ => new ProblemDetails
+      {
+        Title = "Database error",
+        Detail = $"An error occurred while saving data to the database. {postgresException.MessageText}"
+      }
+    };
+
+    logger.LogDebug("Database exception: {message}", context.Exception.CompleteExceptionMessage());
+    context.ExceptionHandled = true;
+    context.Result = new ObjectResult(problemDetails)
+    {
+      ContentTypes = { "application/problem+json" },
+      StatusCode = StatusCodes.Status500InternalServerError
+    };
+  }
+}