feat(backend): ability to set profile flags, return profile flags in get user endpoint

Foxnouns.Backend/Database/FlagQueryExtensions.cs

@@ -0,0 +1,36 @@
+using Foxnouns.Backend.Database.Models;
+using Microsoft.EntityFrameworkCore;
+
+namespace Foxnouns.Backend.Database;
+
+public static class FlagQueryExtensions
+{
+    private static async Task<List<PrideFlag>> GetFlagsAsync(this DatabaseContext db, Snowflake userId) =>
+        await db.PrideFlags.Where(f => f.UserId == userId).OrderBy(f => f.Id).ToListAsync();
+
+    /// <summary>
+    /// Sets the user's profile flags to the given IDs. Returns a validation error if any of the flag IDs are unknown
+    /// or if too many IDs are given. Duplicates are allowed.
+    /// </summary>
+    public static async Task<ValidationError?> SetUserFlagsAsync(this DatabaseContext db, Snowflake userId,
+        Snowflake[] flagIds)
+    {
+        var currentFlags = await db.UserFlags.Where(f => f.UserId == userId).ToListAsync();
+        foreach (var flag in currentFlags)
+            db.UserFlags.Remove(flag);
+
+        // If there's no new flags to set, we're done
+        if (flagIds.Length == 0) return null;
+        if (flagIds.Length > 100) return ValidationError.LengthError("Too many profile flags", 0, 100, flagIds.Length);
+
+        var flags = await db.GetFlagsAsync(userId);
+        var unknownFlagIds = flagIds.Where(id => flags.All(f => f.Id != id)).ToArray();
+        if (unknownFlagIds.Length != 0)
+            return ValidationError.GenericValidationError("Unknown flag IDs", unknownFlagIds);
+
+        var userFlags = flagIds.Select(id => new UserFlag { PrideFlagId = id, UserId = userId });
+        db.UserFlags.AddRange(userFlags);
+
+        return null;
+    }
+}