Musify/backend/Core/Musify.Application/Users/FollowUser.cs

using ErrorOr;
using Mediator;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Logging;
using Musify.Application.Contracts;
using Musify.Application.Shared;
using Musify.Domain.Entities;

namespace Musify.Application.Users;

public record FollowUserCommand(long FollowerId, long FollowedId) : ICommand<ErrorOr<Success>>;

public class FollowUserCommandHandler(
    IDatabase database,
    ILogger<FollowUserCommandHandler> logger)
    : ICommandHandler<FollowUserCommand, ErrorOr<Success>>
{
    public async ValueTask<ErrorOr<Success>> Handle(FollowUserCommand request, CancellationToken cancellationToken)
    {
        if (request.FollowerId == request.FollowedId)
            return Error.Validation(description: "A user cannot follow themselves.");

        var followedExists = await database.Users
            .AsNoTracking()
            .AnyAsync(u => u.Id == request.FollowedId, cancellationToken);
        if (!followedExists)
        {
            logger.LogInformation("User {FollowedId} not found", request.FollowedId);
            return Error.NotFound(description: $"User {request.FollowedId} not found");
        }

        var alreadyFollowing = await database.UserFollows
            .AsNoTracking()
            .AnyAsync(f => f.FollowerId == request.FollowerId && f.FollowedId == request.FollowedId, cancellationToken);
        if (alreadyFollowing)
            return new Success();

        await database.UserFollows.AddAsync(new UserFollow
        {
            FollowerId = request.FollowerId,
            FollowedId = request.FollowedId
        }, cancellationToken);

        // A concurrent double click can hit the unique index; already following is a successful no-op.
        var saved = await database.TrySaveChangesAsync(
            AppErrors.Conflict("UserFollow.Duplicate", "Already following this user."), cancellationToken);
        if (saved.IsError)
            return Result.Success;

        logger.LogInformation("User {FollowerId} followed user {FollowedId}", request.FollowerId, request.FollowedId);

        return new Success();
    }
}